Assuming that the output fits in memory, you can get the output as an array of lines by using qx// aka `` in list context, and then use grep. However, I would recommend using capturex from IPC::System::Simple because there are potential security issues with qx//, or, if the output can become too large to sensibly fit into memory, there are other methods/modules to run the external program and get its output line-by-line, similar to what Discipulus showed. See my post on these topics here (with lots of example code).

use warnings;
use strict;
use IPC::System::Simple qw/capturex/;

my @cmd = ('cat', 'sample.txt');  # just an example
print map { s/^_id://; $_ } grep { /^_id:/ } capturex(@cmd);
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Note that map { s/^_id://; $_ } grep { /^_id:/ } can be written more briefly as grep { s/^_id:// }, although people often recommend against using grep for its side-effects.