This is what grep was designed to do:

@a = grep { not is_unwanted($_) } @a;
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Update: and if you really need to modify the array in place, using splice, then you can iterate on the indices in reverse:

my @a = qw(bob bob alice bob john);

for (reverse 0 .. $#a) {
    splice @a, $_, 1 if $a[$_] eq "bob";
}
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