use strict;
use warnings;
use Data::Dump 'dump';

my @numbers = ( 1, 2, 2, 4, 8, 42, 7, 2, 6, 7, 9, 42 );

my @duplicate_locations = find_duplicates( @numbers );

dump @duplicate_locations;


sub find_duplicates {
    my @list = @_;
    my $idx;
    my %buckets;
    foreach my $item ( @list ) {
        push @{$buckets{$item}}, $idx++;
    }
    my @rv;
    foreach my $key ( keys %buckets ) {
        push @rv, { $list[$buckets{$key}->[0]] => $buckets{$key} }
            if @{$buckets{$key}} > 1;
    }
    return @rv;
}