use strict;
my $str = 'genre: "drama", "thriller", "mystery", "espionage"
genre: "drama", "sci-fi"
NOT genre: "drama", "sci-fi"';
my $regex = qr/(?:\G(?!^)|^genre:\h*").*?\K",\h*"/mip;
my $subst = ', ';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html