You can override CORE::open and close. The following should get you going:

BEGIN {
    *CORE::GLOBAL::open = \&my_open;

    sub my_open(*;$@) {
        my ( undef, $filename, $line ) = caller;
        print "opening $_[2] in $filename:$line\n";
        CORE::open($_[0], $_[1], $_[2]);
    }
}

open my $fh, "<", "temp~" or die $!;
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Googling came up with Testing Legacy Code