The tilde ~ is a unary bitwise operator to negate the bits (ones complement), see perlop.

The actual number returned is all bits set:

printf "0%08x\n",4294967295;
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Gives: 0ffffffff

m returns 0 on a failure to match:

printf "0%08x\n",~0;
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also gives 0ffffffff.

Updated to explain more.