I tried to enable the shader while in world, but it didn't work and the "(fallback)" thing appeared. Then I tried to change the settings of the shader a few times, but it crashed. Here's the log with all the details: https://mclo.gs/rdwvnKI

It works! All three modes. Are there any specific settings that I should test?

Not really, but please do submit another issue if you do end up finding a setting that doesn't work - thanks for all your help, and I'll be posting the patch on Modrinth and CurseForge soon™.

Thanks for submitting a bug report.
Do you still have the settings you used when the shader crashed? You can find them as a text file with the same name as the shader in your shaders folder. Also, which version of the shader were you using? I think you renamed the file so I can't tell which version it was just from the logs.

It would also help to say what graphics card you have, if you know that.

I'm on AMD Ryzen 5 5500U with Radeon Graphics. Shader version 1.3.1, Iris version 1.6.4. Here's the error given by Iris when I enable the shader (settings are on default):

deferred.fsh: deferred.fsh: ERROR: 0:180: 'constructor' : array constructor needs one argument per array element 
ERROR: 0:180: '=' :  cannot convert from ' const float' to ' temp 4-element array of float'
ERROR: 0:180: '' : compilation terminated 
ERROR: 3 compilation errors.  No code generated.

The issue should be fixed in Pixel Perfect 1.3.2, which has just gone live on Modrinth and CurseForge. Unfortunately I didn't have an AMD graphics card on hand, so I had to test the issue on an Intel iGPU. Please reply with whether this fixed it for you so I know if I can close the issue.

Try using this patched file, hopefully it fixes the issue:
Pixel-Perfect-Integrated-Graphics-Patch.zip
Not releasing it yet since I'm not sure if it fixes the issue.

Could you please load the shader in Iris and then attach .minecraft>patched_shaders as a zip file? That would help me a lot since that contains the file that's mentioned in the error and changes depending on which device the shader is loaded on.

Thanks for being so helpful, by the way

Same error

patched_shaders.zip

Yes, if I enable your file with the Iris config option enableDebugOptions=false. If I set that to true (in order for the patched_shaders folder to appear), turns out the error line numbers are different:

sky_basic.fsh: sky_basic.fsh: ERROR: 0:285: 'clamp' : ambiguous function signature match: multiple signatures match under implicit type conversion 
ERROR: 0:285: 'assign' :  cannot convert from ' const double' to ' temp 3-component vector of float'
ERROR: 0:285: '' : compilation terminated 
ERROR: 3 compilation errors.  No code generated.

I didn't see that at first because Iris didn't send the usual error message in the chat, so I didn't bother to check

Are you absolutely sure you got the exact same error as last time, including the line numbers?

Pixel-Perfect-Patch-2.zip
Does this change your error at all?