It appears that Athanasius¹ hit the nail on the head... first shot.

Since my foo didn't provide that insight directly, I tried adding use diagnostics; which produced this illuminating comment:

Search pattern not terminated at D:\_Perl_\PMonks\1157546.pl line 7 (#
+1)
    (F) The lexer couldn't find the final delimiter of a // or m{}
    construct.  Remember that bracketing delimiters count nesting leve
+l.
    Missing the leading $ from a variable $m may cause this error.

    Note that since Perl 5.9.0 a // can also be the defined-or
    construct, not just the empty search pattern.  Therefore code writ
+ten
    in Perl 5.9.0 or later that uses the // as the defined-or can be
    misparsed by pre-5.9.0 Perls as a non-terminated search pattern.

Uncaught exception from user code:
        Search pattern not terminated at D:\_Perl_\PMonks\1157546.pl l
+ine 7.
[download]

The confirmation there of your surmise should lead pretty directly to (somewhat obscure) documentation (that I didn't bother to chase down because you already had a valid answer from the Reverend Canon.

JBTW, my error line as cited by diagnostics is off-by-one because I add a node number to the test code.

Update ¹ ...and choroba who offered similar wisdom as I was preparing this.