Seems like a lot of extra work:

$doctext =~ s/(a href=")([^"]+)/$1 . link_check($2)/eg;

sub link_check {
    my $link = shift;
    # $hostname is a global or block lexical
    return $link if ($link =~ /^http://$hostname/);
    $link =~ s!(http://[^/]+)?!$hostname!;
    return $link;
}
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Disclaimer: this is untested.

Update: Changed the second $1 on the RHS to $2, as originally intended.