# need if, or it'll warn on no match

No need: no substitution is done, no capture variables are referenced, if no replacement match exists:

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $s ('G1 Z0.050 F7800.000',  'G10 ; retract',) {
   printf qq{'$s' -> };
   local $_ = $s;
   s/(G1 Z)(\d{1,2}\.\d{3})/$1-$2/;
   print qq{'$_'};
   }
"
'G1 Z0.050 F7800.000' -> 'G1 Z-0.050 F7800.000'
'G10 ; retract' -> 'G10 ; retract'
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(Although I think I would prefer to use look-arounds as graff did and just stick a - in.)