#! perl -slw
use strict;

my @files = @ARGV;

my @fhs;
my $i = 0;

open $fhs[ $i++ ], '<', $_ or die "$_ : $!" for @files;

#while( my @lines = map{ scalar <$_> || () } @fhs ) { !!Bad code!!

while( ## Build an array of one line (or the null string) from each file.
	my @lines = map{ 
		defined( my $line =<$_> ) 
			? $line 
			: '' 
	} @fhs 
) {
	chomp @lines; ## Remove the newlines
	print join ' ', @lines; ## and concatenate them.
}

close $_ for @fhs;

__END__
P:\test>465737 test1.dat test2.dat test3.dat
file1 line 1 file2 line 1 file3 line 1
file1 line 2 file2 line 2 file3 line 2
file1 line 3 file2 line 3 file3 line 3
file1 line 4 file2 line 4 file3 line 4
...