Are you sure? 

for ( 1 .. 5 ) {
    $foo = qr/$_/o;
    print "Match!\n" if '1' =~ m/$foo/;
}

[download]

and 

for ( 1 .. 5 ) {
    $foo = qr/$_/;
    print "Match!\n" if '1' =~ m/$foo/;
}

[download]

print entirely different outputs on perl 5.8.8 for me.

Further, the reference in perlop for qr lists this usage:

qr/STRING/msixpo

The 'o' is listed as meaning "Compile pattern only once.".

perlop does go on to say that if the compiled regex is embedded in a larger pattern that 'o', unlike the other options, will not propagate out to the other parts of the regex.

In reply to Re^2: Precompiling qr/.../o question by mr_mischief
in thread Precompiling qr/.../o question by vit

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