Perl does not use the C runtime's copy of the environment. Probably for very good reasons.

Rather than accessing the CRTs copy from XS, you should access Perl's copy instead.

This prints the value of the pre-existing envvar 'path', then nothing from the (probably) non-exitant envvar 'fred', before setting 'fred' from Perl and then retrieving it from XS.:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'XSenv', CLEAN_AFTER_BUILD => 0;

char *test( char *name  ) {
    HV *env = get_hv( "ENV", 0 );
    SV **pval = hv_fetch( env, name, strlen( name ), 0 );

    if( pval ) {
        SV *val = *pval;
        return( SvPVX( val ) );
    }
    return( "" );
}

END_C

print test( 'path' );
print test( 'fred' );

$ENV{fred} = 12345;

print test( 'fred' );
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