1. A set of unambiguous requirements describing exactly what this program is supposed to do
2. A set of example inputs
3. A set of expected outputs

No code. Just those documents.

ok, thanks, 1) basically open file "data" which contains 2 columns of numerical data approx 400 lines. Copy this data to another file "complete"

e.g. file "complete"

0  1
1  0
1  0

e.g. file data
0.521 0.365
0.258  0.456
0.211 0.912

so output would be:
0  1  0.521  0.365
1  0  0.258  0.456
1  0  0.211  0.912
[download]

thanks,

So, your basic set of steps is:
1. Read file1
2. Read file2
3. Write file3 with each line from file1 and file2

What happens if they don't have the same number of lines?

chomp( my @file1 = do { open FH, (+shift); <FH> } );
chomp( my @file2 = do { open FH, (+shift); <FH> } );

my @file3;
while( @file1 && @file2)
{
    push @file3, join(' ', shift(@file1), shift(@file2));
}

open FH, ">output";
print FH join($/, @file3, '');
close FH;
[download]

Of course, you're going to want to add some error-handling for the file operations.

------
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Then there are Damian modules.... *sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon. - flyingmoose

I shouldn't have to say this, but any code, unless otherwise stated, is untested