print "ps before=$param_string\n";
    my @ef = (extract_multiple($param_string,
                             [
                               sub { extract_delimited($_[0]) },
                               sub { extract_bracketed($_[0]) },
                               sub { extract_variable($_[0]) },
                               qr/([^,]+)(.*)/,
                             ],
                              undef,1));
    print "extracted=@ef\n\n";

Results:
ps before=0.0, 0.0, 0.0, @inches, 0.0, 0.0, [0.0,0.0,'PI$pxtr19i'] 
extracted=0.0

ps before=0.0,0.0,0.0,@inches,0.0,0.0,[0.0,0.0,'PI$pxtr19i'] 
extracted=0.0

ps before= "TERMINAL_DRILL_SIZE", "", , @scale , , [0.034, 0.0]
extracted="TERMINAL_DRILL_SIZE" ""  


If I remove the regexp match like this:
    my @ef = (extract_multiple($param_string,
                             [
                               sub { extract_delimited($_[0]) },
                               sub { extract_bracketed($_[0]) },
                               sub { extract_variable($_[0]) },
                               #qr/([^,]+)(.*)/,
                             ],
                              undef,1));

then the results are:
ps before=0.0, 0.0, 0.0, @inches, 0.0, 0.0, [0.0,0.0,'PI$pxtr19i'] 
extracted=@inches [0.0,0.0,'PI$pxtr19i']

ps before=0.0,0.0,0.0,@inches,0.0,0.0,[0.0,0.0,'PI$pxtr19i'] 
extracted=@inches [0.0,0.0,'PI$pxtr19i']

ps before= "TERMINAL_DRILL_SIZE", "", , @scale , , [0.034, 0.0]
extracted="TERMINAL_DRILL_SIZE" "" @scale [0.034, 0.0]