What i would suggest is making an array with a list of files. Code Untested

my @files = qw( file1 file2 );
my @userNames = qw( chris mike );

&WriteMyFiles(\@userNames,\@files);

sub WriteMyFiles
{
    my $userNames = shift(@_);
    my $files = shift(@_);

    foreach my $file ( @$files ) {
       open(OUT, ">> $file") or die("Cannot Open $file\n"); 
        foreach my $USER(@$userNames){
            print OUT "$USER\n";
        }
        close(OUT);
    }

}
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