Actually, in that respect, Perl's regexp engine does handle mutiple "m//g" instances properly. Witness the following code:

use strict;
use warnings;
use diagnostics;

my( $first, $second ) = ( "a1b2c3d4e5f6", "AaBbCc" );

while( $first =~ m/(\d)/g ) {
    print "\$first matched on $1\n";
    while( $second =~ m/([a-z])/g ) {
        print "\t\$second matched on $1\n";
    }
}
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You might be thinking of this quote, from perltodo:

A re-entrant regexp engine
This will allow the use of a regex from inside (?{ }), (??{ }) and (?(?{ })|) constructs.