It's in the XS file (I don't have the sources of it, just the libryry file).
I'd think it's like in Filter::decrypt, where it goes like this: 
BOOT:
    /* Check for the presence of the Perl Compiler */
    if (gv_stashpvn("B", 1, FALSE))
        croak("Aborting, Compiler detected") ;

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In reply to Re^2: Breaking a Filter::decrypt like source filter by olego
in thread Breaking a Filter::decrypt like source filter by olego

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