If I have a script that "uses" multiple Perl modules, and I know that a function named f was imported from one of them, i.e. it was not defined in the script itself

Don't do that. Be explicit about what you import.

use Coneheads qw( GnarfleTheGarthock );
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This can’t be emphasized enough.   A couple of years ago I was wrestling with a really old application which “inexplicably” did not run on one of several different machines.   It turned out to be a problem such as this:   there was a common subroutine-name, more than one potential source of it, and a very slight difference in the file setup on that one machine ... a file was there but not being picked up, long story.   But the bottom line was that the intended (correct ...) behavior of that code as-written was in effect contextually determined, with an ambiguity in play that was hiding the root problem.   Learning from this, I now try to be as explicit as possible in every place where such ambiguities might occur: using qw() import-lists with large “kitchen sink” utility packages, and sometimes empty lists there with explicit fully-qualified (i.e. with package name expressly specified) references to the exact routine that I need.   These problems, when I have seen them, come from software that was originally written for older Perls.

$ cat script.pl 
use Carp;
carp(777);

$ perl -MO=Xref script.pl
File ..../linux/x86_64/5u5/lib/perl5/5.12.2/Carp.pm
  Subroutine Carp::carp
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Thanks to ysth and his response to the question "How to de-reference a coderef? (B tricks)", I came up with the following function:

sub SourcePackage { my($funcname, $package) = @_;

$package = caller() if !$package;
return if ! $package->can($funcname);

my $coderef;
{ no strict 'refs'; $coderef = \&{"$package\:\:$funcname"}; }
return B::svref_2object($coderef)->GV->STASH->NAME;
} # SourcePackage()
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So, in the situation I described, where a script imports from multiple Perl modules, e.g. "use X; use Y; use Z;", and you want to know which module a function $funcname was imported from, you call find out with:

my $srcPkg = SourcePackage($funcname, __PACKAGE__);
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which will return the name of the package that $funcname was imported from (and into package __PACKAGE__ - i.e. the current package), or undef if no function by that name is in the symbol table of package __PACKAGE__ (which in the case of a script without a package declaration, will be 'main').

FYI, the { and } around "no strict 'refs';" was to restrict the "no strict" to the smallest code possible. The function SourcePackage() can be put into an external library of utilities, which is why I created it so that the package name must be passed in - however, by default, if you don't pass in a package name, it will use the package that the SourcePackage() subroutine was called from, which I'm sure would be the most common use case. So, the above call can be abbreviated to:

my $srcPkg = SourcePackage($funcname);
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