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;
}

##</code><code>##

({ 42 => [5, 11] }, { 7 => [6, 9] }, { 2 => [1, 2, 7] })

##</code><code>##

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;
}

##</code><code>##

{ 2 => [1, 2, 7], 7 => [6, 9], 42 => [5, 11] }