Answer to SO Q62653173

so_q62653173.pl

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

use List::Util;

sub replace_first_keys {
    my ( $hash, $replace, $replacement ) = @_;
    unshift @$replace, $replacement if exists $hash->{$replacement};
    $hash->{$replacement} = {
        map { %{ delete $hash->{$_} } }
          grep { ( exists $hash->{$_} ) && ( ref $hash->{$_} eq 'HASH' ) }
          ( List::Util::uniq @$replace )
    };
    $hash;
}
# REPLACE FIRST KEYS OF $hash LISTED IN @$replace WITH THE KEY $replacement 
# POSSIBLE USAGE:
# - `replace_first_keys( \%Hash1, [ 3, 4 ], 300 );` (present use case)
# - `replace_first_keys( \%Hash1, [ 3, 4 ], 2 );` (replacement key exists, older values are overwritten on conflict)
# - `replace_first_keys( \%Hash1, [ 3, 4 ], 4 );` (replacement also in replace list, handles this properly)

# TODO ANSWER https://stackoverflow.com/q/62653173/1921546

use Data::Dumper;

my %Hash1;
$Hash1{1}{12}  = 1;
$Hash1{1}{10}  = 1;
$Hash1{2}{31}  = 1;
$Hash1{3}{52}  = 1;
$Hash1{3}{58}  = 1;
$Hash1{4}{82}  = 1;
$Hash1{4}{154} = 1;

print Dumper( \%Hash1 );

replace_first_keys( \%Hash1, [ 3, 4 ], 300 );

print Dumper( \%Hash1 );