Thanks for all your replies. In the end I went for fletch's suggestion to localise $ENV{ TZ }, because I needed to compute the difference between local time and the remote time and could not permit the code to switch to remote time throughout. Another factor in choosing the most convenient solution was that it was easier to use core modules only rather than require module installation all the way down the production line (dev -> test -> QA -> uat -> prod.) for so small a piece of code.

Update: OK to be honest I did not explicitly use 'local' because the tzset had to be repeated anyway. What I actually did was more like:

sub RemoteTime {
    my $realtz = $ENV{ TZ };
    my ( @return, $return );
    my $wantarray = wantarray;
    $ENV{ TZ } = shift;
    tzset;
    if ( $wantarray ) {
        @return = localtime();
    }
    else
        $return = localtime();
    }
    $ENV{ TZ } = $realtz;
    tzset;
    return $wantarray ? @return : $return;
}
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