May I offer

#!

use Smart::Comments;
use strict;
use warnings;

my $sql=<<"__SQL__";
/* comment! */
create table `table name`( # another comment!
   `field name1` type1 not null default 'x has a blank or two',
   `field name2` type2 default null,
#   ...
   primary key (`pk name`),
   key `key name` (`field name1`, `field name2`)
) engine=other things here;
__SQL__
### $sql

my $RegExp_s=qr{(?: 
        (`(?:[^`]|\`)*?`)       # backticked string
        |(?:\/\*.*?\*\/)        # /* */ comments
        |(?:'(?:[^']|\')*?')    # single quoted strings
        |(?:"(?:[^"]|\")*?")    # qouble quoted strings
        |(?m:\#.+?$)            # single line # trailing comments
         )}sx; #'               # . includes \n

while ($sql =~ m{$RegExp_s}g) {
    if (defined $-[1]) { # backticked string 
        substr($sql,$-[1],$+[1]-$-[1])=~ s/ /_/g;
         };
     };
### $sql
__END__
[download]

which yields

### $sql: '/* comment! */
create table `table name`( # another comment!
   `field name1` type1 not null default \'x has a blank or two\',
   `field name2` type2 default null,
#   ...
   primary key (`pk name`),
   key `key name` (`field name1`, `field name2`)
) engine=other things here;
'

### $sql: '/* comment! */
create table `table_name`( # another comment!
   `field_name1` type1 not null default \'x has a blank or two\',
   `field_name2` type2 default null,
#   ...
   primary key (`pk_name`),
   key `key_name` (`field_name1`, `field_name2`)
) engine=other things here;
'
[download]

Commented the regex.

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