use strict;
while (<DATA>) {
    chomp;
    print "$_ is ", /^(?:19|20)
                      (?:
                        (?:(?<=19)[5-9]|(?<=20)[0-4])[0-9] |
                        50)$
                    /x ? "Ok" : "not ok", "\n"
}

__DATA__
1050
1950
2050
2054
1980
2004
3100
[download]

Updated: Added the trailing '$' in the regex to limit the length to 4 characters. Thanks MCS for pointing that out. :-)