What is the meaning of operator ||=

Question

In Perl, what's the meaning of operator ||= in the following example ?

$sheet -> {MaxCol} ||= $sheet -> {MinCol};

Answer 1

a ||= b is similar to a = a || b, so:

$sheet->{MaxCol} ||= $sheet->{MinCol};

is similar to:

$sheet->{MaxCol} = $sheet->{MaxCol} || $sheet->{MinCol};

Per ikegami's comment, the difference is that a ||= b; only evaluates a once, and it evaluates a before b. This matters when a is magical or isn't a scalar.

Answer 2

$sheet -> {MaxCol} ||= $sheet -> {MinCol};

have same effect as

if (!$sheet->{MaxCol}) { $sheet->{MaxCol} = $sheet->{MinCol}; }

or

$sheet->{MaxCol} = $sheet->{MinCol} unless $sheet->{MaxCol};