This code results in a syntax error at the end of the here-doc. The problem is that Perl doesn't know about dequote yet. Making the dequote usage more explicit (&dequote, dequote()) doesn't help in this case.

It has nothing to do with dequote or prototypes. You have essentially written

my $foo = '
...
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which results in a syntaxt error (Can't find string terminator "'" anywhere before EOF in this case, " END" in yours).