Does $a match the regex pattern in $b?

$a =~ /$b/
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Does $a contain the string in $b?

$a =~ /\Q$b\E/

$a =~ /\Q$b/  # \E is optional

my $b_re = quotemeta($b);  # This is what \Q does
$a =~ /$b_re/
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Does $a equal the string in $b?

$a eq $b

$a =~ /^\Q$b\E\z/

my $b_re = quotemeta($b);
$a =~ /^$b_re\z/
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quotemeta

First, avoid using $a and $b; they're special.

If you are intending for $regexPattern to be a user supplied regex that may contain errors, then perhaps you want to *eval* the regex match and trap said errors.

my $matched;
my $contender = 'foo';
my $regexPattern = '+foo';  #won't work
eval { $matched = $contender =~ /$regexPattern/ }; warn $@ if $@;
print "Hey, did I survive the error? Yay!\n";
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First, $a and $b are probably just dummy variables to indicate the problem. And even if they do, most of the time, there's no problem at all when using $a and $b.

But if we're nitpicking, if you shouldn't use a construct just because in some cases, this fails, you made a bigger boo-boo than the op with his $a and $b. You're trying to use $@ as an indication whether an eval failed or not. That's wrong; an eval may fail while $@ is false due to cleanup actions. The only way to be sure an eval succeeded is to check its return value:

eval {
    $matched = $contender =~ /$regexPattern/;
    1;
} or warn $@ // "Unknown error";
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