use strict;
my $str = '<p class="2"><span class="CharOverride-7">A book title</span><a href="https://aaa.net"><span class="CharOverride-8">https://aaa.net</span></a><span class="CharOverride-7">.</span></p>
<p class="2"><span class="CharOverride-7">Another book title</span><a href="https://aaa.net"><span class="CharOverride-8">https://aaa.net/</span></a><span class="CharOverride-7">.</span></p>
';
my $regex = qr/<span class="CharOverride-7">(.*)<\/span>/p;
my $subst = '$1';
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