Untested, and I'm not sure about some of the variables.

use File::Temp 'tempfile';
use File::Copy;
use Mail::Address;

my $datafile = 'some file';

open( my $fh, $datafile ) or die "Cannot read $datafile: $!\n";

my ($out, $newfile ) = tempfile();

my %addresses = map {
    my ($user, $domain) = Mail::Address->parse( $_ );
    $domain => $user;
} 'bob@domain1.com', 'bob@domain2.com';

my $domains = qr( '(' . join('|',
    map { quotemeta( $_ ) } keys %addresses) . ')' );

while (<$fh>)
{
    print $out $_;
    print $out "$addresses{ $1 }\@$1\t$addresses{ $1 }\n"
        if /$domains/;
}

close( $out );
close( $fh );

move( $newfile, $datafile );
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