The following will work correctly if the year is specified as 0, 1, 11, 111, or even 1111 (ones representing any digit):

use strict;
use warnings;

my( @dates ) = ( '4-10-31', '14-11-31' );

foreach my $date ( @dates ) {
    my $string = $date;
    $string =~ s/^(\d+)/
        my $year=2000;
        substr($year,4-length($1),length($1))=$1;
        $year/e;
    print $string, "\n";
}
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You can even override '2000' as being the base year by including the entire four-digit year, as in "1987-10-31".