use CGI;

my $q = new CGI; # Get a CGI object
open (OUT, ">$q->param('outfile')") || die $!; # Open our output file
my @files = grep{/^file\d+$/} $q->param(); # Grab a list of params
for my $filename (@files){
    open (IN, $q->param($filename)) || die $!; # Open each input file
    print OUT <IN>; # Print the input file to the output file
    close (IN); # Make sure you close the input file
}
close (OUT);