read-stdin.pl

#!/usr/bin/perl

while(<STDIN>){ # Puts each line at $_ , which can be used in the loop.
	chomp; #Implicitly uses $_, if no parameter is specified.
	print "hello $_\n"; # Uses $_ explicitly in a string.
}

## It is possible to do a fast little trick on the command line:
# perl -pe '#Transform $_#'
##  -n : surround you code with 'while (<>) { #your code# }' .
##  -p : surround you code with 'while (<>) { #your code# }' and print $_ each loop.
##  -e : execute code.
##  -a : split $_ words to the variable @F .
## This way, perl can be used kind like sed.
##
## See example below, which filters the words, printing only the first word of each sentence:
# $ perl -pe 's/\s*(\w+).*/$1/;' << EOF
# > There is a pie.
# > No pie.
# > Where pie?
# > EOF
# There
# No
# Where
##
## Go check https://www.cs.cf.ac.uk/Dave/PERL/node164.html