Nothing seems to beat the obvious for efficiency in my mind:-

my @input=('a','b','c','d');
my $char='b';
# --------------
my $pos = -1;
ELM: for ( my $i = 0; $i <= $#input; $i++ ) {
    if ( $char eq $input[$i] ) {
        $pos = $i;
        last ELM;
    }
}
[download]

with $pos now having -1 for no match or 0+ for where the match was made in the array.