some_unique_key other stuff I want to grab
another_key more other stuff that's cool
wow_how_many_keys_are_there lot's - why do you ask?

##</code><code>##

#!/your/perl/here

use strict;
use warnings;

my %file1;
my $key;
my $value;

usage() unless @ARGV == 3;

my $output_file = pop @ARGV;

# read first file
while (<>)
{
	chomp;
	($key,$value) = split / /, $_, 2
	  or warn "Bad data on line $. in file $ARGV, ";
	$file1{$key} = $value 
	  or warn "Bad data on line $. in file $ARGV, ";
}
continue
{
	# reset line numbers for warning messages
	# end loop
	if ( eof ) # note special form of eof
	{
		close ARGV;
		last;
	}
}

while (<>)
{
	chomp;
	($key,$value) = split / /, $_, 2
	  or warn "Bad data on line $. in file $ARGV, ";
	if ( exists( $file1{$key} ) )
	{
		$file1{$key} .= ' ' . $value
		  or warn "Bad data on line $. in file $ARGV, ";
	}
	else
	{
		warn "Can\'t find key matching <$key> (line $.) " 
		  . "in file <$ARGV>, ";
		$file1{$key} = $value
		  or warn "Bad data on line $. in file $ARGV, ";
	}
}		
continue
{
	last if ( eof ) # note special form of eof
}

open( OUT, ">", $output_file )
  or die "Error opening $output_file for writing, ";
  
foreach my $k ( keys %file1 )
{
	print OUT "$k $file1{$k}\n";
}

sub usage
{
	die "Need 3 filenames on the command line.\n" 
	  . "First 2 files are merged by key into the 3rd file.\n";
}

__END__

##</code><code>##

one one one one one
two two two two two
three three three three

##</code><code>##

one ONE ONE ONE ONE
two TWO TWO TWO TWO
three THREE THREE THREE

##</code><code>##

three three three three THREE THREE THREE
one one one one one ONE ONE ONE ONE
two two two two two TWO TWO TWO TWO