OP writes:

1) while(<stdin>){print " I saw $_" ;} # Is thie (sic) equivalent to 2) while($_=<stdin>){ print "I saw $_" ;} so how will while loop end because undefined value in the $_ will not let the loop end

In Perl, the assignment $_ = <> doesn't necessarily make $_ "defined"? Strange isn't it? But here is how it happens:

 my $x = undef;   #clears out $x and makes it undefined
 my $y = $x;      #since $x is undefined, so is $y
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It might be easier to think of "undefined" in Perl as "null" rather than "not yet assigned".

Also, it isn't necessary for $_ to be undefined (or unassigned) for the loop to end. A while loop ends when expr in while (expr) evaluates to false. Being undefined is only one way of being false. Other ways of being false are:

• being equal to 0
• being equal to the empty string, ''

Best, beth