my $str   = "update patch-5.1.1.21510"
my $patch =        "patch-5.1.1.21510.gpg";

if( $str =~ /$patch/ms ) { print "Your Perl is broken!!!\n"; }

##</code><code>##

my $str = "update patch-5A1B1C21510\ngpg"; # ;)

##</code><code>##

my $patch = "patch-5.1.1.21510";
if( $str =~ m/\Q$patch\E/ ) { .... }