I am going to take a guess that given sort do_nothing(@in), do_nothing is being parsed as the name of the sort sub, and the (@in) is being parsed as the list to sort on. That's what it appears to be doing from the Deparse output, anyway.

Yes, that's exactly what Deparse shows, and that's my point: why is perl behaving this way. Perhaps the following variant makes the point more forcibly. I have redefined do_nothing to behave like a typical sort sub; the output now is sorted, but note that the sort sub (do_nothing) is being put to work with the syntax do_nothing( @in ):

use strict;

sub do_nothing { $a cmp $b }

my @in = qw( e n i g m a );

my @out = sort do_nothing( @in );

print "@out\n";

__END__

% perl enigma.pl
a e g i m n
[download]

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