while(<DATA>){
    my $orig = $_;
    if ( s/foo\@there\.com/bar\@here\.com/ ) {
        print "< $orig> $_\n";
    }
}
__DATA__
foo@there.com
john.foo@there.com
foo@there.com.au
<a href="mailto:foo@there.com">Mr Foo</a>

##</code><code>##

< foo@there.com
> bar@here.com

< john.foo@there.com
> john.bar@here.com

< foo@there.com.au
> bar@here.com.au

< <a href="mailto:foo@there.com">Mr Foo</a>
> <a href="mailto:bar@here.com">Mr Foo</a>

##</code><code>##

s{([a-zA-Z_\.]+\@[a-zA-Zz\.\-]+)}
 {$1 eq $old ? $new : $1}ge