ruby - can't modify frozen string (TypeError)

Question

Got

... '[]=': can't modify frozen string (TypeError)

when trying to modify what I thought was a copy of ARGV[0].

Same results for each of

arg = ARGV[ 0 ]
arg_cloned = ARGV[ 0 ].clone
arg_to_s = ARGV[ 0 ].to_s

arg[ 'x' ] = 'y'
arg_cloned[ 'x' ] = 'y'
arg_to_s[ 'x' ] = 'y'

Answer 1

since google took too long to find the right answer ...

needed to do

arg_dup = ARGV[ 0 ].dup

Answer 2

ok, why is ARGV frozen?