try single quoting your command line string

# cat x           
use warnings;
use strict ;
for my $line (split /\\n/, shift) {
   print "$line\n" ;
}
for my $line (split /\n/, "x\ny\nz") {
   print "$line\n" ;
}

# perl x 'a\nb\nc'
a
b
c
x
y
z
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