open FH, "first file" || die "problem with first file : $!";
@first =<FH>; # slurp into array
close FH;
open FH, "next file" || die "problem with first file : $!";
@second =<FH>; # slurp into array
close FH;
# at this point, we've got the lines of each file in two different arrays.
# if it's important that they have the same number of lines, you can 
# check with if (scalar (@first) != scalar (@second)) {...
# 
    
# now start output to the third file. maybe check to see if it exists?
open OUTPUT, ">output.txt";
# since we're reading from 2 different arrays
# it's easier to use for (0..$#first) rather than foreach (@first)
# so that we can apply the element number to both arrays.
# 
my $sep=","; # specifies comma separated output. this might be bad if your data has commas in it.
 
for (0..$#first) { # this is a liability if the second file has more lines...    
    print OUTPUT join $sep, split (/\s+?/, $first($_)), split (/\s+?/, $second($_));   
}
close OUTPUT;