I wouldn't use a prototype here. They're much more headache than they're usually worth.

sub indices {
    my $match = shift;

    my $i = 0;
    map { $i++; $_ eq $match ? $i : () } @_
}
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Returning an empty list in the map callback causes that iteration to disappear from the result list. This is useful because you can use it as a surrogate grep that allows you to return something other than what your input was.

Update: removed ->[0] copy paste remainder from code.

Update 2: the following is more idiomatic and pretty much makes having a separate sub useless. We're back to grep too:

sub indices {
    my $match = shift;
    grep $_[$_] eq $match, 0 .. $#_
}
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Makeshifts last the longest.