sub find_year {
    my( $str ) = @_;
    my $year;
    $year = $1
        if( $str =~ /\b((?:19|20)\d\d)\b/ );
    return $year
}


for my $title (
    "Let's party like it's 1999",
    "If 6 was 9",
    "If 6 was 9",
    "Summer of 69",
    "Disco 2000",
) {
    print $title , " => ", find_year($title), "\n";
}