#! /usr/bin/env perl
use strict;
use warnings;

use 5.010;

use Data::Dumper;

my $path = '/path/to/dir';

my %files_by_ext;

opendir my $dh, $path or die "opendir($path) failed: $!";

for my $entry ( readdir $dh ) {

    next if $entry =~ m/^\.\.?$/;

    my $ext = ( split m/\./, $entry )[-1];

    next unless $ext;

    # push filename to HoA, key is filename's extension
    push @{ $files_by_ext{$ext} }, $entry;
}

closedir $dh;

# check hash-of-arrays
say Dumper \%files_by_ext;

__END__