$regex = "href\\s*=\\s*\".*?\"";
$regex = 'href\s*=\s*".*?"';

##</code><code>##

use strict;
use warnings qw( FATAL all );

my $regex = 'href\s*=\s*".*?"';
print "regex = $regex\n";
foreach my $line (<DATA>) {
    chomp($line);
    while ($line =~ /$regex/) {
        my $match = $&;
        $line =~ s/$match/xxx/;
    }
}
__DATA__
No anchors here
<a href="asdf">go there</a> <a href="fdsa">go here</a>