unpack doesn't support those sizes directly. You could extract them in parts, then assemble them.

# 3-bytes, network order

my ($o1, $o2, $o3) = unpack('CCC', $_);
my $n = ($o1 << 16) | ($o2 << 8) | $o3;

   -or-

my ($n1, $n2) = unpack('Cn', $_);
my $n = ($n1 << 16) | $n2;
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Alternatively, you could add padding.

# 3-bytes, network order

my $n = unpack('N', "\x00".$_);
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