The part that you may be missing is that '$_' is not created by <STDIN> - it's created as a result of the 'while' loop, in the same way that it would be created if you'd used a 'for' loop.

while (<STDIN>){          # Loops over <STDIN> as long as it returns '
+true'
    print;
}

while ($_ = <STDIN>){     # Exact equivalent of the above but "spelled
+ out" explicitly
    print $_;             # Ditto for this line - exactly equvalent to
+ a simple 'print;'
}

for (1,2,3){              # Loop over specified list
   print;                 # Print out each item (which has been assign
+ed to $_) in turn
}

for $_ (1,2,3){           # Exact equivalent of the above but "spelled
+ out" explicitly
   print $_;              # Same story
}
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In general, whenever you see a variable being omitted (e.g., 's/foo/bar/' instead of '$xyz =~ s/foo/bar/'), '$_' is being used. It's part of the standard Perl idiom, and you need to get familiar with it. At the very least, you'll need it to read other people's code.

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells