$ cat t.pl
use strict; use warnings;

while (<DATA>) {
    my @f = split /\s+/,$_;
    $f[1] =~ s/foo/bar/g;
    print "result: $f[1]\n";
}

__DATA__
Poor foolish kitty

The empty line above this one might fool us.

$ perl t.pl
$ perl t.pl
result: kitty
Use of uninitialized value $f[1] in substitution (s///) at t.pl line 6, <DATA> line 2.
Use of uninitialized value $f[1] in concatenation (.) or string at t.pl line 7, <DATA> line 2.
result:
result: barlish
Use of uninitialized value $f[1] in substitution (s///) at t.pl line 6, <DATA> line 4.
Use of uninitialized value $f[1] in concatenation (.) or string at t.pl line 7, <DATA> line 4.
result:

##</code><code>##

while (<DATA>) {
    next if /^\s*$/;  # skip blank lines
    ...
}