#!usr/bin/perl -w
use strict;

while (<DATA>)
{
   if(/\balpha\b/)
   {
       print "term 'alpha' detected on input line $.\n";
       next;             
   }
   .. do something here...
   .. sometimes better than elsif()
}

__DATA__
beta
beta
beta
alpha # this line appears only once in the whole file
beta
beta
beta

##</code><code>##

while ( <FILE> ) 
{
    if ( /alpha/ ) 
    {
        ....
        next;
    }
    if (/beta/)
    {
        ....
        next;
    }
    #here is the default code...
}