Your validation tests look reasonable. One optimisation would be to use an array for the error messages, like so:

# untested, change to suit
my @messages;
if ( ....test.... )
{
    push @messages, "The error message";
}
# more tests

PrintError if @messages;

sub PrintError
{
    # any initial html stuff, or use a template
    print join('<br>', @messages);
}
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...which means you don't need to maintain a flag.

Looking on CPAN, I see CGI::Validate which appears to a a Getopt style validator and Params::Validate which may be useful.

Update:Podmaster suggested Data::FormValidator, which is what I was trying to find.

Update2: I left the & in there because that's what the supplicant had - now removed. For reasons why not to do this, see perlsub. Basically, with the &, the argument list to a sub is optional and you will get the @_ array visible at the time of calling in the subroutine.