The first pass gathers the filenames in %file. The second pass handles the files suffix per suffix if an handler of the form handle_XXX is defined for the suffix XXX. Note that you can add new handlers without touching the logic.

#! /usr/bin/perl 
use strict;
my %file;  # hash of filenames arrays keyed by suffix

# first pass
for (<*>) {
    push @{$file{$2}}, $_  if /(.*\.(.*))/;
}

# second pass
for my $suffix (sort keys %file) {
    no strict 'refs';
    next unless defined &{"handle_$suffix"};
    &{"handle_$suffix"}($_) for   sort @{$file{$suffix}} 
      
}

# process the file with suffix '.cpp'
sub handle_cpp {
    print "$_[0]\n";
}
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-- stefp