Quoting is tricky.

next LINE if length $line == 1 and q{\} eq substr $line, 0, 1;

That's a compile error. I think you want q{\\}

Readonly my $NEWLINE => q{\n};

use Readonly;

{
# I don't think you want this
Readonly my $NEWLINE => q{\n};
print ">>>$NEWLINE<<<\n";
}

{
# Do you really want this?
Readonly my $NEWLINE => qq{\n};
print ">>>$NEWLINE<<<\n";
}

__END__

>>>\n<<<
>>>
<<<
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