while (<DATA>) {
   ($first) = $_ =~ m/^(\S+)/;
    print "$first\n";
}

__DATA__
This is a line.
Another line is here.
  Here's a line.

Line five is this one.

##</code><code>##

while (<DATA>) {
    $first = (split/\s/)[0];
    print "$first\n";
}

##</code><code>##

while (<DATA>) {
    $first = substr $_, 0, index $_, " ";
    print "$first\n";
}