$line = "jlla jldsfjlfj lfla (1\p=n-\5) fk (2) ljljj ljlfaufi  lfjlakfh (10, 11) lfjdlaj (10, 11\p=n-\15) ndsfdjksnk (2, 15, 16) dkhfak";

##</code><code>##

$line = "jlla jldsfjlfj lfla (<lr=C1>1</lr>\p=n-\<lr=C5>5</lr>) fk (<lr=C2>2</lr>) ljljj ljlfaufi  lfjlakfh (<lr=C10>10</lr>, <lr=C11>11</lr>) lfjdlaj (<lr=C10>10</lr>, <lr=C11>11</lr>\p=n-\<lr=C15>15</lr>) ndsfdjksnk (<lr=C2>2</lr>, <lr=C15>15</lr>, <lr=C16>16</lr>) dkhfak";

##</code><code>##

$line=~s!\(([1-9]{1,3}((, |\\p=n-\\)[1-9]{1,3})*)\)!\(@{[map"<lr=C$_>$_</lr>",split"(, |\\p=n-\\)",$1]}\)!g;

##</code><code>##

Unmatched ( in regex; marked by <-- HERE in m/( <-- HERE , |\p=n-\)/ at s.pl line 8.