This module provides three utility subroutines that make it easier to
develop interactive applications...

* 'is_interactive()'

This subroutine returns true if '*ARGV' and the currently selected
filehandle (usually '*STDOUT') are connected to the terminal. The test is
considerably more sophisticated than:

-t *ARGV && -t *STDOUT

as it takes into account the magic behaviour of '*ARGV'.

You can also pass 'is_interactive' a writable filehandle, in which case
it requires that filehandle be connected to a terminal (instead of the
currently selected). The usual suspect here is '*STDERR':

if ( is_interactive(*STDERR) ) {
carp $warning;
}

* 'interactive()'

This subroutine returns '*STDOUT' if 'is_interactive' is true. If
'is_interactive()' is false, 'interactive' returns a filehandle that does
not print.

This makes it easy to create applications that print out only when the
application is interactive:

print {interactive} "Please enter a value: ";
my $value = <>;

You can also pass 'interactive' a writable filehandle, in which case it
writes to that filehandle if it is connected to a terminal (instead of
writinbg to '*STDOUT'). Once again, the usual suspect is '*STDERR':

print {interactive(*STDERR)} $warning;

* 'busy {...}'

This subroutine takes a block as its single argument and executes that
block. Whilst the block is executed, '*ARGV' is temporarily replaced by a
closed filehandle. That is, no input from '*ARGV' is possible in a 'busy'
block. Furthermore, any attempts to send input into the 'busy' block
through '*ARGV' is intercepted and a warning message is printed to
'*STDERR'. The 'busy' call returns a filehandle that contains the
intercepted input.

A 'busy' block is therefore useful to prevent attempts at input when the
program is busy at some non-interactive task.