c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s (qw(a_________  %%a_____%%  123456789_)) {
   print qq{'$s' matches} if $s =~ /[[:alnum:]]+[_]+/;
   }
"
'a_________' matches
'%%a_____%%' matches
'123456789_' matches