@listA = ('This=That', 'Some=Song');
@listB = ('Other=That', 'Corny=Song');

##</code><code>##

$text =~ s/.+?=/$new_text=/;

##</code><code>##

foreach (@listA, @listB) {   # or just one of the two, whatever
   # set $new_text to whatever you want
   s/.+?=/$new_text=/;
}

##</code><code>##

s/.+?(?==)/$new_text/;