#!/usr/bin/perl

use strict;
use warnings;

# get the list of file names -- actually, just make sure @ARGV has them:
# declare a hash for output

die "Usage: $0 *.txt > text.joined\n" unless ( @ARGV and -f $ARGV[0] );
my %output;

# extra step: declare an array to preserve original order of keys in first file
my @output_order;

# open the first file
# while reading each line from the file
#    get the first column of the line for use as a hash key
#    assign the line as the value of hash element using that key

open( IN, '<', $ARGV[0] ) or die "Can't read $ARGV[0]: $!\n";
while (<IN>) {
    my ( $key ) = ( /^(\S+)/ );

#  extra steps: add key to order array, turn EOL whitespace into tab character
    push @output_order, $key;
    s/\s+$/\t/;

    $output{$key} = $_;
}

shift @ARGV;  # (extra step: removes the file name that we just handled)

# for each remaining file
#    open the file
#    while reading each line from the file
#        get the first column of the line for use as a hash key
#        append the line to the current value of the hash element using that key

for my $file ( @ARGV ) {
    open( IN, '<', $file ) or die "Can't read $file: $!\n";
    while (<IN>) {
        my ( $key ) = ( /^(\S+)/ );

# extra step: turn EOL whitespace into a tab character
        s/\s+$/\t/;

        $output{$key} .= $_;
    }
}

# for each key in the hash -- using the original ordering
#     print the value of the hash element using that key

for my $key ( @output_order ) {

#  extra step: convert the final tab character into a line-feed
    s/\t$/\n/;

    print $output{$key};
}