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;
}