Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

2.2 Inserting Code with eval

The Skipper
can save disk space (and brainspace) by bringing the definition for
turn_towards_heading out into a separate file. For
example, suppose the Skipper figures out a half-dozen common
subroutines related to navigating the Minnow that he seems to use in
most or all of the programs he's writing for the
task. He can put them in a separate file called
navigation.pl, which consists only of the needed
subroutines.

But now, how can you tell Perl to pull in that program snippet from
another file? You could do it the hard way:

sub load_common_subroutines {
open MORE_CODE, "navigation.pl" or die "navigation.pl: $!";
undef $/; # enable slurp mode
my $more_code = <MORE_CODE>;
close MORE_CODE;
eval $more_code;
die $@ if $@;
}

The code from
navigation.pl is read into the
$more_code variable. You then use
eval to process that text as Perl code. Any
lexical variables in $more_code will remain local
to the evaluated code.[1] If
there's a syntax error, the $@
variable is set and causes the subroutine to die
with the appropriate error message.

[1] Oddly, the variable
$more_code is also visible to the evaluated code,
not that it is of any use to change that variable during the
eval.

Now instead of a few dozen lines of common subroutines to place in
each file, you simply have one subroutine to insert in each file.

But
that's not very nice, especially if you need to keep
doing this kind of task repeatedly. Luckily, there's
(at least) one Perl built-in to help you out.