Quoting perldoc -f undef:

       undef EXPR
       undef   Undefines the value of EXPR, which must be an lvalue.  
+Use only
               on a scalar value, an array (using "@"), a hash (using 
+"%"), a
               subroutine (using "&"), or a typeglob (using "*").  (Sa
+ying
               "undef $hash{$key}" will probably not do what you expec
+t on
               most predefined variables or DBM list values, so don't 
+do that;
               see delete.)  Always returns the undefined value.
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So, your if, while undef'ing $nomSQL, returns undef. And you can't perform a s/// substitution on an undefined variable.

Are you sure you don't want:

...

You know, I was going to post some code, but I really can't tell what you want to do. Obviously you want to 'escape' single quotes in an SQL statement, but it looks like you're trying your darndest to make sure the string is blank before you do it.

What do you want to do?

$,=42;for(34,0,-3,9,-11,11,-17,7,-5){$*.=pack'c'=>$,+=$_}for(reverse s
+plit//=>$*
){$%++?$ %%2?push@C,$_,$":push@c,$_,$":(push@C,$_,$")&&push@c,$"}$C[$#
+C]=$/;($#C
>$#c)?($ c=\@C)&&($ C=\@c):($ c=\@c)&&($C=\@C);$%=$|;for(@$c){print$_^
+$$C[$%++]}
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