What you have there does not compile

syntax error at Module.pm line 5, near "$\("
BEGIN not safe after errors--compilation aborted at Module.pm line 5.
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I presume your code is closer to

package Module;
use strict;
use warnings;
BEGIN {
    my $pathname = $INC{"$\(__PACKAGE__)}.pm"};
    $pathname =~ s/\.pm$/.conf/;
}
1;
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If so, the problem is that

"$\(__PACKAGE__)}.pm"
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is the same as

$\ . "(__PACKAGE__)}.pm"
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because $\ is a variable. You want

my $pathname = $INC{__PACKAGE__ . ".pm"};
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Except that will fail if your module name has more than one part (e.g. "Foo::Bar"), and it's more complicated that it needs to be. You really just want

my $pathname = __FILE__;
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