In order to make your use conditional, you need to wrap it, and the if condition, in a BEGIN block. This will then get run at compile time (use time).

BEGIN {
    if ( $online == 1 ) { # This variable is defined before, according
+ to the $^0
        use POSIX 'locale_h';
        setlocale( LC_ALL, "pt_BR.iso88591" );
    }
}
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