I'd do it in the following way, observing that the order of matches does not matter:

my @months = (qw(jan feb mar apr may jun jul aug sep oct nov dec));

my $i = 1;
my %month_num = map { $_ => sprintf '%02d', $i++ } @months;

my $month_re = join "|", @months;

$month =~ s/($month_re)\w*/$month_num{$1} || "$1"/ei;

[download]

Update: Fixed bug noted by jwkrahn below.

Update2: Fixed bugs noted by pc88mxer - the OP wanted a two digit number, and also changed \w+ to \w* because otherwise jan won't match anymore

In reply to Re: Quick search-and-replace month names for numbers by Corion
in thread Quick search-and-replace month names for numbers by kangaroobin

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