To avoid using "real" files for demonstration purposes I used Perl's facility for using a string as a file by passing a reference to the string into the open. To open a real file instead you should:

open my $in, '<', $fileName or die "Unable to open $filename: $!";
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Please follow my advice and use strictures (use strict; use warnings; - see The strictures, according to Seuss), the three parameter form of open and lexical file handles (the 'my $in' bit in my sample code). These tips will save you time in the future!

If you open an output file ($out) before the loop in my sample code you can change the print to:

    printf $out $format, $key, @{$data{$key}}{@columnNames};
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to print to the output file instead of STDOUT. Note that for testing and sample code using STDOUT is often much more convenient!

My mistake. It now works beautifully. May I ask for a last favor, though? I want to keep the header -- first line in the first file. I looked at the code to see if I can find where to skip reading the first line, but I could not figure out. So the working code written by GrandFather:

#!/usr/bin/perl
use strict;
use warnings;

my $data1 = "/PRBB/input.txt";
my $data2 = "/PRBB/input2.txt";
my $data3 = "/PRBB/output.txt";

my %data;
my @columnNames;

#open my $in, '<', \$data1;
open my $in, '<', $data1 or die "Unable to open $data1: $!";
push @columnNames, parseFile (\%data, $in);
close $in;

#open $in, '<', \$data2;
open my $in2, '<', $data2 or die "Unable to open $data2: $!";
push @columnNames, parseFile (\%data, $in2);
close $in2;

my $format = (('%-9s ') x (@columnNames + 1)) . "\n";

open my $out, '>', $data3 or die "Unable to open $data3: $!";

for my $key (sort keys %data) {
    next if keys %{$data{$key}} != @columnNames;
    
    printf $out $format, $key, @{$data{$key}}{@columnNames};
}

sub parseFile {
    my ($dataRef, $inFile) = @_;
    my $header = <$inFile>;
    my ($keyColumn, @columns) = map {chomp; split} $header;

    while (defined (my $line = <$inFile>)) {
            chomp $line;
    
        my ($key, @data) = split /\s+/, $line;
        @{$dataRef->{$key}}{@columns} = @data;
    }
    
    return @columns;
}
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Hint: look for a variable called '$header'!

It doesn't seem likely that you do actually want that header though - it doesn't include column titles for the extra data from the other file.

---

True laziness is hard work

Thanks a lot, GrandFather. Now I get the following errors while it does not print out anything to the file. "my" variable $in masks earlier declaration in same scope (line 19) use of uninitialized value $key in hash element (line26) use of uninitialized value $key in printf (line26)

#!/usr/bin/perl
use strict;
use warnings;

my $data1 = "/DATA/input.txt";
my $data2 = "/DATA/input2.txt";
my $data3 = "/DATA/output.txt";

my %data;
my @columnNames;
my $key;

[line 19]open my $in, '<', $data1 or die "Unable to open $data1: $!";
push @columnNames, parseFile (\%data, $in);
close $in;

open my $in, '<', $data2 or die "Unable to open $data2: $!";
push @columnNames, parseFile (\%data, $in);
close $in;

my $format = (('%-9s ') x (@columnNames + 1)) . "\n";

open my $out, '>', $data3 or die "Unable to open $data3: $!";
(line26)printf $out $format, $key, @{$data{$key}}{@columnNames};


for my $key (sort keys %data) {
    next if keys %{$data{$key}} != @columnNames;
    
    printf $format, $key, @{$data{$key}}{@columnNames};
}

sub parseFile {
    my ($dataRef, $inFile) = @_;
    my $header = <$inFile>;
    my ($keyColumn, @columns) = map {chomp; split} $header;

    while (defined (my $line = <$inFile>)) {
            chomp $line;
    
        my ($key, @data) = split /\s+/, $line;
        @{$dataRef->{$key}}{@columns} = @data;
    }
    
    return @columns;
}
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It prints out the output on dos-screen, though. (but not into file)