But you should use anchors in your regex, otherwise it might match, where you don't want it to match

my $x = '5.1.1';


# OOPS
if ( $x =~ m/5.1/ ) {
  print "'$x' is 5.1\n";
}

# better
if ( $x =~ m/^5.1$/ ) {
  print "'$x' is 5.1\n";
}
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update: please read on at FunkyMonks post