in reply to the -x command line parameter
Larry Wall first came to the attention of the wider computing world as the author of patch and rn. The latter, the niftiest usenet news reader of its day, is rarely seen now, but patch is everywhere. In the days when programs were passed around in shar (shell archive) files via usenet, patches (diffs) were generally sent in usenet email, too. Larry made patch ignore anything that didn't look like a diff so you could pipe the message (out of rn, naturally) to patch and have it Do The Right Thing.
Though I can't prove it, (without working harder than I care to on a Sunday night) I'm pretty sure the -x switch in Perl harks back to this use of patch on usenet. Not that it isn't still useful, as others have noted.
"Even if you are on the right track, you'll get run over if you just sit there." - Will Rogers
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Seekers of Perl Wisdom