The Default Variable $_

The default input and pattern-searching variable, used in:

• file tests (-f, -d) except -t which defaults to STDIN
• print, unlink
• pattern matching m// s/// tr/// when used without =~
• default iterator in foreach
• default implicit iterator for grep and map.
• the destination for <FH> when it is the sole criterion of a while test.

# input

while(<>){...} # only equivalent in while
while( defined( $_ = <>)){...}

# input and pattern matching

while( <FILE>){ print if /http:/;}