use CGI:
my $q = new CGI;

use constant BUFFER_SIZE    => 16_384;

my $filename  = $q->param( "filename" ) || die;

# Assuming $filename is sanitized...

my $fh = $q->upload( "file" );
my $buffer = '';

open(OUT,">/tmp/$filename");

binmode $fh;
binmode OUTPUT;

# Write contents to output file
while ( read( $fh, $buffer, BUFFER_SIZE ) ) {
    print OUTPUT $buffer;
}