To answer the why part of your question:

This code was maybe written in this way by the original coder because he/she got used to the pattern of testing a value for definedness before using it in order to avoid the "Use of uninitialized value" warning:

$ perl -wMstrict -e 'my $x; print length $x'
Use of uninitialized value in print at -e line 1.
$ perl -wMstrict -e 'my $x; print length $x if defined $x'
$
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or:

$ perl -wMstrict -e 'my $x; print "yes" if $x=~/foo/'
Use of uninitialized value $x in pattern match (m//) at -e line 1.
$ perl -wMstrict -e 'my $x; print "yes" if defined $x && $x=~/foo/'
$
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However, in this particular case, the test for definedness is not needed:

$ perl -wMstrict -e 'my $x; print "yes" if defined $x && length $x'
$ perl -wMstrict -e 'my $x; print "yes" if length $x'
$
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This is because the length function is being used in a boolean context, and whether it returns 0 or undef doesn't matter since they are both evaluated as a false value, see Truth and Falsehood.