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

# don't do this:
#  print " Enter the number. \n";
...
    unless ( @ARGV == 1 and $ARG[0] =~ /([\d.]+)/ );

my $df = $1;

    your_script_name 10.5 > outfile.name

#!/usr/bin/perl

...
        print join( "\t", @vals ), "\n";
    }
}

 script_name  10  input.txt > 10_x_50_x_input.txt