I created this code a few days ago. It's pretends to be the constant module and overrides constants to what you like. It's mostly a proof-of-concept but you can try it.
# file: constant.pm
package constant;
use warnings;
use strict;
use File::Spec;
my %Overrides;
sub make_inc_hook
{
# Make sure we don't load this file... or we'll recurse infinitely
+ later.
my ($path) = ( grep { -f $_ }
map { File::Spec->catfile( $_, 'constant.pm' ) }
grep { $_ ne q{.} } # avoids our directory
@INC )
or die q{couldn't load constant.pm};
open my $fh, '<', $path or die "open: $!";
# Filter the real constant.pm, rename package to old_constant.
my $source_filter = sub {
return 0 unless defined $_; # edit: added 'defined'
s/package constant;/package old_constant;/;
return 1;
};
# Returns an @INC hook. See perldoc -f require.
return sub {
my ($self, $file) = @_;
return unless $file eq 'old_constant.pm';
return ($fh, $source_filter);
};
}
BEGIN { push @INC, make_inc_hook(); }
# Preload/compile the original constant.pm
use old_constant;
sub import
{
my ($pkg) = caller;
if ( ref $_[1] eq 'HASH' ) {
my $param = $_[1];
$Overrides{ $_ } = $param->{ $_ } foreach keys %{ $param };
return;
}
my (undef, $name, $value) = @_;
$value = $Overrides{ $name } if $Overrides{ $name };
use Data::Dumper;
local $Data::Dumper::Terse = 1;
eval qq{package $pkg; use old_constant '$name' => } . Dumper( $val
+ue );
die if $@;
return;
}
1;
# file: A.pm
package A;
use strict;
use constant VALUE => 1;
sub func {
print "Constant is ", VALUE, "\n";
}
1;
#!/usr/bin/perl
# file: example.pl
use warnings;
use strict;
use lib q{.}; # because constant.pm above is in the same dir...
use constant { VALUE => 1_000_000 }; # this overrides VALUE
use A;
A->func();
edit: Run 'perl example.pl' and the output is: Constant is 1000000
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