EXPR->{EXPR} is a shorthand for ${EXPR}{EXPR}. Normally, the first EXPR would be a scalar holding a reference, but Perl also support symbolic references. Symbolic references allow variable names to be strings.

>perl -le"$x = "var"; ${$x} = "abc"; print $var;"
abc
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Furthermore, barewords are treated as strings literals by default (unless they refer to a known function or a file handle is expected where the bareword is located).

>perl -le"$x = main; print $x"
main
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Using symbolic references is strongly dissuaded. use strict 'refs'; (and therefore just use strict;) prevent them from being used.

Using barewords as string literals is also strongly disuaded. use strict 'subs'; (and therefore just use strict;) prevent them from being used.

So in short, had you used use strict; as you should have, trying to run your snippet would have failed for two different reasons.