Strings, numbers and many other things can be placed in a scalar. Not all scalars are numbers. Not all scalars are strings. Perl will implicitely convert back and forth as needed, possibly resulting in warnings.

print 2+3;       # Number converted to string.
print 'a'.3;     # N->S
print '2'+3;     # S->N, N->S
print 'z'+3;     # S->N (warn), N->S
print 3+undef;   # U->N (warn), N->S
print ''.undef;  # U->S (warn)
print \$a;       # R->S
print \$a+0;     # R->N->S
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