in reply to How can I re-program the exists function?

In a nutshell, you can't.

In the same way, you cannot program your own expression form of map or grep (eg. @squares = map $_**2, 1 .. 10;).

Many of perl's built-ins cannot be directly emulated with Perl code.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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The start of some sanity?

In Section Seekers of Perl Wisdom