Maybe use looks_like_number(sv) prior to calling sv_2nv ... 

#!/usr/bin/perl

use Inline C => <<'END_C';

void num_test(SV* sv) {

     I32 is_num = looks_like_number(sv);
     printf("'%s' looks like a number: %s  (SvNV(): %g)\n",
              SvPV_nolen(sv),
              is_num ? "yes":"no",
              SvNV(sv) ); 
}

END_C

num_test('aa');
num_test('123');
num_test('123aa');
num_test('aa123');
num_test('1.23');

__END__
'aa' looks like a number: no  (SvNV(): 0)
'123' looks like a number: yes  (SvNV(): 123)
'123aa' looks like a number: no  (SvNV(): 123)
'aa123' looks like a number: no  (SvNV(): 0)
'1.23' looks like a number: yes  (SvNV(): 1.23)

[download]

Update: added SvNV() results for comparison.

In reply to Re^5: Why does sv_2nv on non-number look like an NV by almut
in thread Why does sv_2nv on non-number look like an NV by mje

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