Of course when I tried it all sorts of error messages came up

Er ...

print "pre if() statement\n"

if(@ARGV == 0) {
  my $n = 1;
  while(<DATA>) {
    print $n++, $_;
  }
}

print "post if() statement\n";

__DATA__
this
totally
works
[download]

When the above is run it outputs

pre if() statement
1: this
2: totally
3: works
post if() statement
[download]

The point being, of course you can put a while statement or any other looping construct in a block if statement, and vice versa. Blocks in perl can take any and all valid code, as they are more or less just delimiters of scope.

What were the errors you were getting when you had the while in the if?