First always use strictures (use strict; use warnings;). That will tell you that \a, \d, \# and \h are not what you expect. It would also tell you that you are redefining $key which is probably not what you meant.

You seem to be confusing the roles of $key and $line.

Your regex matches a bunch of weird characters. It may be that you mean to match the complement of the set, in which case the set should start [^.

You would likely get much better answers if you told us what the problem is that you are trying to solve and showed us a little more of the code. It looks like you are trying to manipulate paths - you've already been told several times about modules that will help you with that.

The array contents are
...
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I think one or more monks, in replying to one or more of your earlier posts, has mentioned modules like File::Spec and File::Basename. Have you looked into those? Have you tried them? Is there some problem that you have with trying them?

Others have also told you about using split -- e.g. like this:

my @a = qw/C:\abc\dfg\axy C:\abc\dfg\#@$#@$@\hwllo/;
for ( @a ) {
    my @p = split /\\/;
    print "last-component= ", $p[-1], " and next-to-last= ", $p[-2], "
+\n";
}
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But, Perl being the TIMTOWTDI language that it is, I'd like to point out to a different approach, and that is using the modules File::Spec (or its procedural variant File::Spec::Functions), and possibly File::Basename for extracting the full directory and path and the base filename, from a path.

Specifically, you could use abs2rel, like this:

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

in: C:\abc\dfg\#@$#@$@\hwllo
out: #@$#@$@\hwllo
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Furthermore, if you're interested in getting the top level directory (?) name from that result, you can use the function splitdir from File::Spec::Functions:

# 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
+;
}
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in: C:\abc\dfg\axy
out: axy
top directory: axy

in: C:\abc\dfg\#@$#@$@\hwllo
out: #@$#@$@\hwllo
top directory: #@$#@$@
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Do you perhaps mean to get the path-component, just after the --what you call-- key? It just happens to be the last and next-to-last component in your examples, but that is just sheer co-incidence.

The following code will extract the next path-element after the key:

use strict;
use warnings;
use 5.012;

my $key= quotemeta 'C:\abc\dfg'; # we must escape all meta-characters

while (<DATA>) {
    chomp;
    if (/$key\\([^\\]+)/) {
        say "$_ : $1";
    }
    else {
        say "$_ : no match";
    }
}

__DATA__
C:\abc\dfg\axy
C:\abc\dfg\#@$#@$@\hwllo
C:\abc\dfg
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C:\abc\dfg\axy : axy
C:\abc\dfg\#@$#@$@\hwllo : #@$#@$@
C:\abc\dfg : no match
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