I'd use or and not instead of || && ! because the long names:

• are better readable, and especially the ! is often "hidden behind other code".
• have the lowest precedence which might avoid errors if the expressions get more complicated, especially if there are .. ... ?: = += -+ and so on , => within them. Especially stuff like open FH, $filename || die "Error: $!"; is fatal.

So I'd write it the following way, although it is a little bit longer:

print "no editor found\n"
    unless defined $ENV{EDITOR} or defined $ENV{VISUAL};
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