#!/usr/bin/env perl -l

use strict;
use warnings;

# Set these constant values
my %month_num_for = qw{ 
    January 1 February 2 March 3 April 4 May 5 June 6
    July 7 August 8 September 9 October 10 November 11 December 12
};
my $format = '%4d%02d%02d-%s';

# Get this data from file
my $old_filename = 'abc001';
my $long_date = 'January 5, 2002';

# Split up the long format date
my ($month, $day, $year) = split /\,?\s+/, $long_date;

# Generate the new filename
my $new_filename
    = sprintf $format, $year, $month_num_for{$month}, $day, $old_filename;

# For testing
print "New filename: $new_filename";

# When you're happy with the testing
# rename $old_filename => $new_filename