From Time::Zone (which is the same as Time::Timezone iirc):

$TZ = defined($ENV{'TZ'}) ? ( $ENV{'TZ'} ? $ENV{'TZ'} : 'GMT' ) : ''
    unless $TZ;
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Where $TZ is taken as the first argument (to the functions needing a base timezone). =)

So, in other words, you can pass it a value and it does not rely on there being a TZ environment variable.