because Perl may choose between different extensions.

.plc , .pmc and .pod come to mind.

I doubt that perl would create a list of directories and grep for those files that have matching extensions. Simply because it is too much work. It is much easier to concat the base filename and the extensions one after the other, until one can be opened. Roughly like this:

# perlish pseudo-code, too lazy for C today
sub tryLoadMod
{
    my $basename=shift;
    for my $ext (qw(pm pmc plc pod aua wtf)) {
        if (open my $handle,'<',"$basename.$ext") {
            compileAndRun($handle);
            last;
        }
    }
}
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Alexander