The /o is hardly an optimization, and potentially dangerous. /o prevents recompilation of the regexp. However, since the regexp doesn't change, as it's not interpolating a variable, perl already knows the regexp hasn't changed, so no recompilation happens.

Where it does make a difference is in situations like this: 

for my $word ("foo", "bar", "baz") {
    if ($line =~ /quux $word/o) {yada yada yada}
}

[download]
Now using the /o modifier prevents the regexp from being recompiled. It's faster. It's unlikely to be correct though.

The only use for /o is the slight optimization where you have a regexp with an interpolating variable and where the variable doesn't change. Without /o perl takes the time to check whether the interpolated string has changed, and recompiles only when it has. With /o, perl skips this check. Typically, the check is dwarved by the execution of the regexp, but there are a few cases where it's a tiny optimization. Given the dangers of /o, I prefer to use qr instead.

IMO, the use of /o should be a mandatory warning, asking the programmer if he really, really wants this. Because for every correct use of it I have seen, I've seen 100 incorrect uses, and 1000 programmers who don't know what /o does.

In reply to Re^4: stripping whitespace gives an 'unitialized value' error by JavaFan
in thread stripping whitespace gives an 'unitialized value' error by stevemayes

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