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Subroutines
In Perl, a programmer can name a block of code which can be executed by simply calling out that name. These named blocks of code are called subroutines and look like this (the line numbers in this example are for reference only).

     1 $a = &mysub;            # $a is assigned the sub return value 
     2 $a = &mysub (1,2,3);    # three arguments passed to sub
     3 sub mysub {             # subroutine definition
     4   $arg1 = @_[0];        # not recommended
     5   ($arg1, $arg2) = @_;  # common way to read arguments
     6   print 'hello';
     7   return 7';            # value 7 is returned to calling expression
     8 }                       # end of subroutine code block
     9 $a = mysub;             # & is optional since sub now exists

A subroutine is called simply by treating the subroutine as a function whose name starts with & - as shown in line 1 of the code example above.

Subroutine Location
A subroutine may be placed anywhere in the Perl program. If a line of code calls a subroutine that is found later in the program, the & is required. If the subroutine precedes the line of code that calls it, the & is optional. However, it is considered good practice to use the & at all times.

Arguments
When a subroutine is defined, no arguments are included in the definition. When calling a subroutine, however, multiple values may be passed to the subroutines, as in line 2 of the example above. The arguments are made available within the special array variable @_. above. Any of the Perl array functions may be applied to the @_ variable to access the arguments.

If @ array is modified directly, the variables from the main scripts that were used to pass values into the subroutine are also changed. But if you copy elements (line 4) into local variables within the subroutine, the variables in the main script are not modified.

Return Values
When a subroutine is called it returns the value of the last expression evaluated. Or you may use a return statement (line 7) to explicitly specifiy a return value. A return statement without a value returns undef.

Variable Scope
Global variables may be accessed within a subroutine. Also, variables defined in subroutines are global unless declared using 'my'. If not declared using 'my', subroutine variables may be accessed from the main program.

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