The standard module Time::Local will do the trick.

use Time::Local;
my $date = ' 19990230'; # 30th Feb 1999
$date =~ s/\s+$//;
$date =~ s/^\s*//;
my ($year, $month, $day) = unpack "A4 A2 A2", $date;
eval{ 
    timelocal(0,0,0,$day, $month-1, $year); # dies in case of bad date
+ 
    1;
} or print "Bad date: $@";
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UPD: But this won't work for valid dates less than Jan 1 1970 and greater than Jan 19 2038. If you need to check dates outside this range, try Date::Manip.