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

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

dump { find_duplicates( @numbers ) };


sub find_duplicates {
    my @list = @_;
    my $idx;
    my %buckets;
    foreach my $item ( @list ) {
        push @{$buckets{$item}}, $idx++;
    }
    delete @buckets{ grep { @{$buckets{$_}} < 2 } keys %buckets };
    return %buckets;
}