A one-liner:

perl -ple"printf q[%s], scalar <STDIN> unless $n++ % 10" file1 < file2
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Homework?

LOL - No, not homework. I am performance testing a system that uses multiple data repositories. In Production, about 10% of requests get serviced by system A, and 90% get serviced by system B. To properly mimic production I need to load test the system with similar requests, hence the 10/90 split.

OK. But doing a "1 line from file A followed by 9 lines from file B", you are actually handling a degenerate case where all requests are artificially spaced in an even way. Wouldn't it be more "real" to randomly determine from which file to read the next request:

if (int(rand(10) = 0)) {
    # read from file A
} else {
    # read from file B
}
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CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

I'm with others here wondering what possible purpose this serves, but I would probably do something like the below. $in_file1 being the larger of the two files.

open($in_fh1, '<', $in_file1);
open($in_fh2, '<', $in_file2);
open($out_fh, '>', $out_file);

while (my $line1 = <$in_fh1>)
{
   chomp($line1);
   state $i;
   $i++;
   say {$out_fh} $line1;
   unless ($i % 10)
   {
      chomp(my $line2 = <$in_fh2>);
      say {$out_fh} $line2 if $line2;
   }
}
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I'd recommend a simpler code:

open my $in_fh1, '<', $in_file1;
open my $in_fh2, '<', $in_file2;
open my $out_fh, '>', $out_file;

while (defined(my $line1 = <$in_fh1>)) {
    print {$out_fh} $line1;
    unless ($. % 10) {
        my $line = <$in_fh2>;
        print {$out_fh} $line;
    }
}
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The use of $. does make things more simple. I'll have to take note of that for similar situations in the future. What is the benefit of the added defined call?

This term: "evenly merge" does not have a clear definition that I know of.

Did you mean: ten rows from File1, then one row from File2?
That seems rather easy - too easy for this question - so what is it that you mean by "evenly merge".

Ooops, update: I guess it really is simple. My bad. Loop for 10 lines from File1 and send each line to File3. Send one line from File2 to File3. Loop back for the next 10 lines from File1. What's the problem?

Say that you read each file into an array.

use File::Slurp;
my @file1 = read_file('file1');
my @file2 = read_file('file2');
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Where would you go from there? Eventually you could have a new array, write it out like this:

my $lines = merge_lines(\@file1, \@file2);
write_file('file3', @$lines);
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How would you write 'merge_lines'?

sub merge_lines {
    my ($lines1, $lines2) = @_;
    my @lines;
    ....
    return \@lines;
}
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Not the most efficient way to do it, but...

  sort -n -k1,1 <(nl -ba -i2 -v1 $FILE1) <(nl -ba -i20 -v0 $FILE2) | c
+ut -f2-
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