I modified your code a little to fit the problem description slightly better:

use strict;

my $num_words = 4;

my @words = ();
open FILE, "<in.txt" or die $!;
while ( <FILE> )
{
        last if ( push ( @words, m/\s*([^\s]+)\s*/g ) >= $num_words );
}
close FILE;

print join( " ", @words ) . "\n";
[download]

As a side note, is there a reason to use [^\s] rather than \S, or is it just a matter of preference? Thanks.