my $pid = open my $fh, '-|', 'sleep 3; echo aaaa' // die $!;
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That will never die because of precedence. (The string 'sleep 3; echo aaaa' is always true.)

$ perl -MO=Deparse -e'my $pid = open my $fh, "-|", "sleep 3; echo aaaa
+" // die $!;'
my $pid = open(my $fh, '-|', 'sleep 3; echo aaaa');
-e syntax OK
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You need to add parentheses:

$ perl -MO=Deparse -e'my $pid = open( my $fh, "-|", "sleep 3; echo aaa
+a" ) // die $!;'
my $pid = open(my $fh, '-|', 'sleep 3; echo aaaa') // die($!);
-e syntax OK
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