for sure davies's answer is wiser but maybe you are interested to know that also the feared eval string can accomplish the taske:

perl -e "$opts = 'io'; $rex = eval qq(qr/aka/$opts); print 'match' if 
+'ALAKAIA' =~ $rex"

match
[download]

PS infact eval is powerful but dangerous: consider at least to check what was passed in following the good principle never trust user input that can be also read as never trust user or just never trust ;=)

my $opts = $ARGV[0];

die "never trust user input" unless $opts=~/^[msixpodualn]+$/;

...
[download]

L*