...
open OUT, ">", "outfile.name" or die "outfile.name: $!\n";
...
print OUT $DNA_conc;
# and so on...

##</code><code>##

# don't do this:
#  print " Enter the number. \n";
#  chomp ($df = <STDIN>);
# do this instead:
die "Usage: $0  multiplier_value\n"
    unless ( @ARGV == 1 and $ARG[0] =~ /([\d.]+)/ );

my $df = $1;

##</code><code>##

    your_script_name 10.5 > outfile.name

##</code><code>##

#!/usr/bin/perl

use strict;
use Regexp::Common;

die "Usage: $0 multiplier_number [input.file]\n"
    unless ( @ARGV and $ARGV[0] =~ /^$RE{num}{real}$/ );

my $df = shift;  # take the numeric arg off @ARGV

while (<>)   # read from STDIN, or from file(s) named in @ARGV
{
    if ( ! /^\s*(?:$RE{num}{real}\s+)+$/ ) {   # if line has non-numerics
        print;
    }
    else {
        my @vals = map { $_ * $df * 50 } split;
        print join( "\t", @vals ), "\n";
    }
}

##</code><code>##

 script_name  10  input.txt > 10_x_50_x_input.txt