#! perl
use strict;
use warnings;
use Tie::File;
use constant
{
    FIELD => 3,     # Number of the field to extract from each record
    SEP   => ',',   # Separator for fields in each output file record
};

my @infile_names = qw( file1.dat file2.dat );
my $outfile_name = 'exp3.dat';

open(my $in, '<', $infile_names[0])
    or die "Cannot open file '$infile_names[0]' for reading: $!";

open(my $out, '>', $outfile_name)
    or die "Cannot open file '$outfile_name' for writing: $!";

<$in>;      # Discard header
print $out $infile_names[0], "\n";

while (my $line = <$in>)
{
    my @fields = split /\s+/, $line;
    print $out $fields[FIELD], "\n";
}

close $out or die "Cannot close file '$outfile_name': $!";
close $in  or die "Cannot close file '$infile_names[0]': $!";

tie my @array, 'Tie::File', $outfile_name
    or die "Cannot tie file '$outfile_name': $!";

for my $file (@infile_names[1 .. $#infile_names])
{
    open(my $in, '<', $file)
        or die "Cannot open file '$file' for reading: $!";

    <$in>;      # Discard header
    $array[0] .= SEP . $file;

    while (my $line = <$in>)
    {
        my @fields = split /\s+/, $line;
        $array[$. - 1] .= SEP . $fields[FIELD];
    }

    close $in or die "Cannot close file '$file': $!";
}

untie @array;