This is how I would do it:

#!/usr/bin/perl

use warnings;
use strict;
use Date::Manip qw(ParseDate);

my @years = (2004..2008);

sub find_year {
    my ($date) = @_;
    for my $year (@years) {
        my $date = ParseDate("$date $year");
        return $year if $date;
    }
    return undef;
}

my @dates = ("Thu Sep 18", "Wed Sep 7");

for (@dates) {
    my $year = find_year($_);
    print "$_ => " . (defined($year) ? $year : "not found" ) . "\n";
}
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