It's trying to read from /dev/tty. You'll either need to run it using something like Expect and feed it via a PTY, or change your code to behave differently depending on where STDIN is coming from.

my $get_a_line = -t STDIN
                 ? sub { readline( $_[0] ) }
                 : sub { scalar <STDIN> };
## . . .
my $input  $get_a_line->( $prompt );
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