A block can only be used where a statement is expected, in a control flow statement where a block is expected, or as an operand to an operator that expects a block.

Some operators that accept blocks:
do BLOCK
map BLOCK LIST
print BLOCK LIST
eval BLOCK

In your code, do would perform what you want

my $foo = do {
    print "hi!\n";
    my $bar = 7;
    my $baz = 8;
};
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