[update]As grep said,[/update] you're sending a full set of headers (and even content) before your code gets to send the redirect header. You need to have things in this order:

#!/usr/bin/perl
use CGI;

my $cgi = new CGI;

if ($cgi->param('smbt')) {
    &creat_cooke();    # sic
    exit print $cgi->redirect('http://example.com/path/Welcome.cgi');
}
print $cgi->header();
# [...]
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