#! /usr/bin/perl
use strict;
use warnings;

# file paths; input for reading, output for writing
my $input1 = 'input1.txt';
my $input2 = 'input2.txt';
my $output = 'output.txt';

# open files; input for reading, output for writing
open my $in1fh, '<', $input1 or die "open($input1,ro) failed: $!\n";
open my $in2fh, '<', $input2 or die "open($input2,ro) failed: $!\n";
open my $outfh, '>', $output or die "open($output,w)  failed: $!\n";

# read linewise from file input1
while ( my $line1 = <$in1fh> ) {
    # read line from file input2
    my $line2 = <$in2fh>;

    # extract email adresses from both lines
    # email is at the beginning of each line and separated with ';' from the rest of the line
    # user@example.org;additional data ...
    my $email1 = ( split m{;}, $line1, 2 )[0];
    my $email2 = ( split m{;}, $line2, 2 )[0];

    # if emails differ, write to output file
    if ( $email1 ne $email2 ) {
        print $outfh, $line1, $line2;
    }
}
# end of file of input1 reached


# if there are lines left in input2, read them linewise and write them to output
while ( my $line2 = <$in2fh> ) {
    print $outfh $line2;
}

# close all files
close $outfh   or die "close($output) failed: $!\n";
close $in1fh   or die "close($input1) failed: $!\n";
close $in2fh   or die "close($input2) failed: $!\n";