use Data::Dumper;

my $base_hash = {};

while (<DATA>) {
    chomp;
    my @keys = split ':';
    my $val = pop @keys;
    my $hashp = $base_hash;
    
    for my $key (@keys) {
        unless (exists $hashp->{$key}) {
            $hashp->{$key} = {};
        }
        $hashp = $hashp->{$key};
    }

    $hashp->{"VAL"} = $val;
}

print Dumper $base_hash;

__DATA__
global:2:3:4:LINE1
global:2:3:4:5:LINE2


########## Output ##########
$VAR1 = {
    'global' => {
        '2' => {
            '3' => {
                '4' => {
                    'VAL' => 'LINE1',
                    '5' => {
                        'VAL' => 'LINE2'
                    }
                }
            }
        }
    }
};
[download]

push @{$hash{globalkey}{key2}{key3}{key4}}, 'ARRAY VALUE';
push @{$hash{globalkey}{key2}{key3}{key4}{key5}}, 'ARRAY VALUE';
#...
[download]

The first line wants the 'key4' entry to be an array, but the second line want that entry to be a hash.

Show some real data and tell us what output you expect.

That's exactly what I was thinking and why I had to make the assumptions I did. I figured, if anything the provided code would help the OP to answer those questions, or at the very least provide a starting position for said OP.

Good catch on pushing the array values. I didn't take that part into my assumptions. So yoda54, got any more details for us?

sub build {
  my $ref  = shift;
  my $keys = shift;
  my $key  = shift @$keys;

  # only one left in keys. that must be the value.
  if ( @$keys == 1 )
  {
    $ref{$key} = $keys->[0];
  }
  else
  {
    $ref{$key} = {}
      unless defined $ref{$key};

    build($ref{$key}, $keys);
  }
}

my $hash = {};
while (<DATA>)
{
  build($hash, split /:/);
}
[download]

  1 use Data::Dumper;
  2 use feature 'say';
  3
  4 my $hashref;
  5 my $code;
  6 while(my $line = <DATA>) {
  7     chomp $line;
  8     my @pieces = split ':',$line;
  9     my $val = pop @pieces;
 10     $code .= ('$hashref'.join '',map { "->{'$_'}" }@pieces)."='$va
+l';\n";
 11 };
 12 eval $code;
 13
 14 print Dumper $hashref;
 15
 16 __DATA__
 17 globalkey:key2:key3:key4:ARRAY VALUE
 18 globalkey:key2:key3:key4:key5:ARRAY VALUE
 19 globalkey:key2:key3:key4:key5:any arbitrary number of keys:ARRAY V
+ALUE

$VAR1 = {
          'globalkey' => {
                           'key2' => {
                                       'key3' => {
                                                   'key4' => 'ARRAY VA
+LUE'
                                                 }
                                     }
                         }
        };
[download]

Because you're using eval dangerously

globalkey:'.`rm -rf /`.':fooled:you: now you data is gone
[download]

you're right , but here it should have worked fine