Just to add a bit, a common performance pitfall is

@regexps = qw( foo bar baz );
@strings = qw( abc def ghi );
for $string (@strings) {
   for $regexp (@regexps) {
      $string =~ /$regexp/
   }
}
[download]

Each regexp is compiled for each element in @strings.

>perl -Mre=debug -e"@regexps = qw( foo bar baz ); @strings = qw( abc d
+ef ghi ); for $string (@strings) { for $regexp (@regexps) { $string =
+~ /$regexp/ } }" 2>&1 | find "Compiling"
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `baz'
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `baz'
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `baz'
[download]

Solution 1: Put the regexp loop on the outside:

@regexps = qw( foo bar baz );
@strings = qw( abc def ghi );
for $regexp (@regexps) {         # All I did was reverse
   for $string (@strings) {      # these two lines.
      $string =~ /$regexp/
   }
}
[download]

>perl -Mre=debug -e"@regexps = qw( foo bar baz ); @strings = qw( abc d
+ef ghi ); for $regexp (@regexps) { for $string (@strings) { $string =
+~ /$regexp/ } }" 2>&1 | find "Compiling"
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `baz'
[download]

Solution 2: Precompile the regexps:

@regexps = map qr/$_/, qw( foo bar baz );   # All I did was add the qr
+//.
@strings = qw( abc def ghi );
for $string (@strings) {
   for $regexp (@regexps) {
      $string =~ /$regexp/
   }
}
[download]

>perl -Mre=debug -e"@regexps = map qr/$_/, qw( foo bar baz ); @strings
+ = qw( abc def ghi ); for $string (@strings) { for $regexp (@regexps)
+ { $string =~ /$regexp/ } }" 2>&1 | find "Compiling"
Compiling REx `foo'
Compiling REx `bar'
Compiling REx `baz'
[download]