This maybe not as short as the former, but i hope itīs clearer. The code creates a tree from the filesystem, that is used to lookup directory names.

Assuming this fs-structure:

test
  ->trEE
    ->/A
      ->/b
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this code:

use strict;
use warnings;

# hash for file-system data
my %tree;

#build a datastructure that represents directory tree
#for everything below "/test"
build_tree ("/test", \%tree);

# get realpath of "c:/test/tree/a/b"
my $p = getRealPath ("TREE/A/B", \%tree);

print $p;

sub build_tree
{ 
    my $dir  = shift;
    my $ref  = shift;
    my $dirh = shift;

    opendir $dirh, $dir or die $!;
    
    while ( $_ = readdir ($dirh) )
    {

        next if /^\./;
        next unless -d "$dir/$_";
        
        $ref->{subdirs}->{lc($_)} = { name=>$_, subdirs=>{} };
        
        build_tree ("$dir/$_", $ref->{subdirs}->{lc($_)});
    }
    
    closedir $dirh;
}

sub getRealPath 
{
    my @path = split ("/", shift);
    my $ref  = shift;
    
    my $path = "";
    
    for ( @path )
    {
        $path .= "/" . $ref->{subdirs}->{lc($_)}->{name};
        $ref = $ref->{subdirs}->{lc($_)};
    }
    
    return $path;

}
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prints:

test/trEE/A/B
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