Proposal to find an answer: write a script using File::Find to iterate over @INC to see whether "use if" is used anywhere and to what purpose. ;)

UPDATE: Here is the script.

use strict;
use warnings;
use File::Find;

find( sub { 
    my $f = $File::Find::name;
    return unless $f =~ /\.p(l|m)$/;
    return unless open my $fh, "<", $f;
    while(<$fh>){
      print "$f: $_" if /^\s*use if/;
    }
    close $fh; }, 
      @INC );
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Just one hit on my machine. "use if" seems to be unpopular.