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 assigned to $_) in turn
}

for $_ (1,2,3){           # Exact equivalent of the above but "spelled out" explicitly
   print $_;              # Same story
}