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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Term::ReadLine reads from the console, not from STDIN. That said, I'm surprised I can't find an option to use STDIO for circumstances such as this one.

I *think* you can do the following:

my $term;
if ($ENV{TESTING}) {
   $term = Term::ReadLine->new('My Application', \*STDIN, \*STDOUT);
} else {
   $term = Term::ReadLine->new('My Application');
}
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or even

my $term;
if (-t STDIN) {
   $term = Term::ReadLine->new('My Application');
} else {
   $term = Term::ReadLine->new('My Application', \*STDIN, \*STDOUT);
}
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The previous might simplify to:

my $term = Term::ReadLine->new('My Application', \*STDIN, \*STDOUT);
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If that doesn't work, the following should (if you only use readline):

use IO::Handle;

{
   package IO::Handle;
   no warnings 'redefine';
   sub readline {
      my ($self) = @_;
      my $line = $self->getline();
      return unless defined $line;
      chomp($line);
      return $line;
   }
}

my $term;
if (-t STDIN) {
   $term = Term::ReadLine->new('My Application');
} else {
   $term = *STDIN;
}
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