The way to check for errors on Net::SFTP::Foreign is calling the error method:

$sftp->put(...);
if ($sftp->error) { ... };
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Or alternatively, use the autodie feature:

my $sftp = Net::SFTP::Foreign->new(..., autodie => 1);
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In order to activate debugging set...

$Net::SFTP::Foreign::debug = -1;
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Also, note that libssh2/Net::SSH2 are somewhat buggy. Ensure that at least you are using the latest versions.