From the psftp script example included with the source:

sub process_ls {
    my($shell, @arg) = @_;
    $shell->mywarn("usage: ls [path]"), return unless @arg < 2;
    $shell->{sftp}->ls($arg[0] || $shell->{pwd},
        sub { print $_[0]->{longname}, "\n" });
}
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If you look at that script it should give you a little more insight on how to use the functions.