Would be nice to honour @INC as use, require, and do. For example:

use File::Spec;            # cdarke added
use Filter::Util::Call;

sub import 
{
    my (undef, @files) = @_;
    
    @files = find_files(@files);    # cdarke added
    
    my (undef, $origin, $line) = caller;
    filter_add sub
    {
        # No change, but see below
        return 1;
    };
}

# cdarke added
sub find_files {
    my @in = @_;
    my @out;
    
    for my $file (@in) {
        my $found = 0;
        for my $dir (@INC) {
            my $path = File::Spec->catfile ($dir, $file); 
            if (-r $path) {
               push @out,$path;
               $found = 1;
               last;
            }
            push @out,$path unless $found;
        }

     }
    
    return @out;
}

1;
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Personally I would prefer to use a lexical file handle, and to have some control over error handling?

Update: corrected inner loop - non-existant files were being ignored.