#!perl -w

use strict;
use File::Find;

find( \&handle_it, '/foo/bar' );

sub handle_it {
    return unless m!\.html\z!;

    my $name = $File::Find::name;

    open( my $fh, '+<', $name )
        or (warn("open on $name failed: $!\n") and return);
    my $content = do { local $/; <$fh> };
    $content =~ s!foo\@there\.com!bar\@here\.us!g;

    seek($fh, 0, 0) or die("seek on $name failed: $!\n");
    truncate($fh, 0) or die("truncate on $name failed: $!\n");

    print $fh $content;

    close($fh) or die("close on $name failed: $!\n");
}