Patterns without interpolation are compiled when their quoting operator (m//, s///, qr//) is compiled. /o shouldn't matter one bit for those, so I won't discuss them.

Patterns with interpolation are compiled when their quoting operator (m//, s///, qr//) is executed.

Perl caches compiled regex patterns to avoid needless recompilation in situations like the following: 

# Compiles each pattern once since m// realises
# you're using the same pattern twice in a row.
for my $re (qw( foo bar )) {
   for (1..2) {
      /$re/
   }
}

[download]

A match or substitution operator can only resuse the last regex is compiled, so the following isn't efficient: 

# Compiles each pattern twice
for (1..2) {
   for my $re (qw( foo bar )) {
      /$re/
   }
}

[download]

You can use qr// to precompile a pattern. 

# Compiles each pattern once
my @res = map qr/$_/, qw( foo bar );
for (1..2) {
   for my $re (@res) {
      /$re/
   }
}

[download]

Note that Perl currently flattens and recompiles compiled patterns interpolated into another pattern. 

# Doesn't recompile $re if it's a qr//.
/$re/

# Stringifies and recompiles $re if it's a qr//,
# but it should be subject to the caching mentioned above.
/x$re/

[download]

This should be optimised in the future.

In reply to Re: Regexp optimization - /o option better than precompiled regexp? by ikegami
in thread Regexp optimization - /o option better than precompiled regexp? by Hessu

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