unless ($fred =~ /^([A-Z_]/w*$/i) {
  print "The value of /$fred doesn't look like a Perl identifier name. /n";
}

if ($fred =~ /^([A-Z_]/w*$/i) {
  //什么都不做
} else {
   print "The value of /$fred doesn't look like a Perl identifier name. /n";
}

if ( ! ($fred =~ /^([A-Z_]/w*$/i) ) {
  print "The value of /$fred doesn't look like a Perl identifier name. /n";
}

#!/usr/bin/perl -w
unless ($mon =~ /^Feb/) {
  print "This month has at least thirty days./n";
} lese {
  print "Do you see what's going on here?/n";
}
#如果用if語句我們可以寫成這樣：
if ($mon =~ /^Feb/) {
  print "Do you see what's going on here?/n";
} else {
  print "This month has at least thirty days./n";
}