my $key = "C:\\abc\\dfg";
my @example = ("C:\\abc\\dfg\\axy", "C:\\abc\\dfg\\#\@\$#\@\$\@\\hwllo");

use File::Spec::Functions qw(abs2rel);
foreach (@example) {
    my $rel = abs2rel($_, $key);
    printf "in: %s\nout: %s\n\n", $_, $rel;
}

##</code><code>##

in: C:\abc\dfg\axy
out: axy

in: C:\abc\dfg\#@$#@$@\hwllo
out: #@$#@$@\hwllo


##</code><code>##

# modified source
use File::Spec::Functions qw(abs2rel splitdir);
foreach (@example) {
    my $rel = abs2rel($_, $key);
    my($topdir) = splitdir($rel);
    printf "in: %s\nout: %s\ntop directory: %s\n\n", $_, $rel, $topdir;
}

##</code><code>##

in: C:\abc\dfg\axy
out: axy
top directory: axy

in: C:\abc\dfg\#@$#@$@\hwllo
out: #@$#@$@\hwllo
top directory: #@$#@$@