I wish to discuss your second example first. undef is a special 'value'. It is the default value given to a scalar when it is created. It can be assigned later, as you have shown. Its behavior depends on context.

As a number, it is treated as zero. AS a string, it is treated as a null string. As a Boolean, it is treated as 'false'. As a argument to defined, it is the only value for which defined will return 'false'.

?perl -e"$line = undef; print 3 + $line"
3
?perl -e"$line = undef; print '3' . $line"
3
?perl -e"$line = undef; print 3 unless $line"
3
?perl -e"$line = undef; print 3 unless defined $line"
3
[download]

Your first example does not do what you think. try it. You probably mean:

$current_time='14:30:05'
(undef, $min, $sec)=split /\:/, $current_time
[download]

I think of this as an idiomatic use which means exactly what you describe. That field of the assignment is ignored.

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