1. use strict; This is a good friend.
2. Declare your variables:  my %dates = ...
3. 01 is an octal number (one). 08 is an illegal number representation. What you want is text, so write text with quotes: my %dates = ( 'Jan' => '01', #...
4. $dates{'Dec'} contains "12". So, forget regex substitution: print "$day $dates{$month} $year\n";
5. There is a lot of caveats with dates. Consider well known CPAN modules like DateTime...

#!/usr/bin/perl

use strict;
my %dates = (   
    'Jan' => '01',
    'Feb' => '02',
    'Mar' => '03',
    'Apr' => '04',
    'May' => '05',
    'Jun' => '06',
    'Jul' => '07',
    'Aug' => '08',
    'Sep' => '09',
    'Oct' => '10',
    'Nov' => '11',
    'Dec' => '12',
);

my $date = "30/Feb/2013";
my ($day,$month,$year) = split '/', $date;
print "$day $dates{$month} $year\n";
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