use warnings;
 use strict;
 
 open my $fh, '+>', 'output.txt'
   or die("Can't open the damn file!: $!");
   
  
while(my $line == 'mytextfile')     # semi-colon removed(run1) & (run2)equity test to '=='
     {                              # extra empty line removed: HazNav (not an error)
     chomp $line;
     my ($name, $num) = split(/\s+(?=\d)/, $line);  # missing semi-colon inserted (run 1)
 
     # some lines have whitespace after the num
 
     $num =~ s/\s+//g;
 
     if ($num =~ /^\d{4}$/){
         $num = "(333)-447-$num";
     }
     elsif ($num =~ /^\d{3}-\d{4}$/){
         $num = "(333)-$num";
     }
 
     print $fh "$name $num\n";
 }

##</code><code>##

C:>perl -c delete_me.pl
Scalar found where operator expected at delete_me.pl line 16, near ")

     # some lines have whitespace after the num

     $num"
        (Missing operator before $num?)
syntax error at delete_me.pl line 8, near ");"
syntax error at delete_me.pl line 16, near ")

     # some lines have whitespace after the num

     $num "
Global symbol "$num" requires explicit package name at delete_me.pl line 16.
Global symbol "$num" requires explicit package name at delete_me.pl line 18.
Global symbol "$num" requires explicit package name at delete_me.pl line 19.
Global symbol "$num" requires explicit package name at delete_me.pl line 19.
Global symbol "$num" requires explicit package name at delete_me.pl line 21.
Global symbol "$num" requires explicit package name at delete_me.pl line 22.
Global symbol "$num" requires explicit package name at delete_me.pl line 22.
delete_me.pl had compilation errors.
##</code><code>##

Found = in conditional, should be == at D:\_Perl_\pl_test\delete_me.pl line 9.