Cryptology A Crash Course in Perl

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$a	»Scalar« variable (number, string). [Perl evaluates the context; explicit deklaration not necessary.]
$a = 5;	Assignment of a number. [Assignments need a closing »;«.]
$a = "Dies ist ein Text.";	Assignment of a string.
$a = <FILE>;	Read a line from a file.
$a = <STDIN>;	Read a line from standard input.
$a = <>;	Read a line from the actual file; default is standard input.
$_ = <FILE>;	Assign line to the »actual variable« $_.
while ($a = <>) {...}	Assign line to the variable $a. Condition is true, if there is a line, false, if end of file is met. [Assignment blocks are enclosed in curly brackets.]
while (<>) {...}	Equivalent with while ($_ = <>) {...}
$a =~ tr/ABC/XYZ/;	Apply a string operator to $a, here tr = »translation operator« = monoalphabetic substitution A --> X, B --> Y, C --> Z
tr/ABC/XYZ/;	Equivalent with $_ =~ tr/ABC/XYZ/;
tr/ABC/$a/;	A --> $, B --> a, C --> $
eval "tr/ABC/$a/";	At runtime first evaluate $a, for example $a = "XYZ"; then execute tr, in the example: A --> X, B --> Y, C --> Z
print $a;	Standard output; equivalent with print(STDOUT,$a);
print;	Equivalent with print $_;

The strength of Perl is in programs that read a (text) file line by line, transform that line in a certain way, and then store the transformed line. A typical illustrative program looks like this:

while (<>) { # Read a line, if there is one left,
  ...;       # transform
  print;     # output.
  }