regular expressions in perl

hello classmates good day,
Can you help me, please, to create a regular expression that prints me n / s: What is a serial number? ;a document?
I tried it in different ways but I do not give it any more

if ($ row = ~ (/[n]/[/:^_]/[s]/ g)
{
push @impresion, ("$ row");

open (EMPLEADOSS, "> serie.txt");

foreach my $ i (0 .. $ # impression) {

print EMPLOYEES $ impression[$i];
}
in the if i've tried

if ($ row = ~ (/ n / s: / g)

my number $ = (n / s :);
if ($ row = ~ (/ $ number / g)

but I do not have any