This question appears a bit strange as normally one would know what the position the "month" is in the string. If that is the case, then no "foreach" would be required. But going with the problem statement...., I would add keys to the month hash with the full month rather than getting a substr of the first 3 letters. Also usually splitting on /\s+/ (the default split) will work out better than splitting on " " because there can be tabs or multiple spaces in the input that are hard to see. As another point, if this line comes directly from input, then splitting on /\s+/ eliminates the need for "chomp;" (\n is a space character).

sub num_month 
{
  my $date_line = shift; #like jan 12 2009 or january 12 2009
    
  my %months  = (JAN => '01', FEB => '02', MAR => '03', 
                APR => '04', MAY=> '05', JUN => '06', 
        JUL => '07', AUG => '08', SEP => '09', 
        OCT => '10', NOV => '11', DEC => '12',
        JANUARY => '01, ....etc.....);
                
  my (@tokens) = split (/\s+/, uc($date_line));
  foreach my $token (@tokens)
  {
     return ($months{$token} if $months{$token});
  }
  die "no month in @tokens";  ## optional but you may want this
}
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Note: if you omit the \n in the die "text", Perl will add the module and line number of the death to the output.

Update:

showing how slit on \s and , can be done and how to get say the 2nd thing in a split via a list slice without using foreach():

#!/usr/bin/perl -w
use strict;

my $date_line = "Jan     2      ,       2009  \n";
my (@tokens) = split (/[\s,]+/, uc($date_line));
print "@tokens\n";
#prints JAN 2 2009

my $date_line2 = "2  jan      ,       2009  \n";
#to get 2nd thing in split, use a list slice...
my $month_text = (split (/[\s,]+/, uc($date_line2)))[1];
print "$month_text\n";
#prints JAN
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