use strict;
use warnings;
use autodie;

open my $users, '<', 'users.txt';

my @files;

for my $file ( qw( file1 file2 file3 ) ) {
    open my $fh, '>>', $file;
    push @files, $fh;
}

while ( my $line = <$users> ) {
    print $_ $line for @files;
}