$ cat b.pl 
use warnings;
use strict;
print "Hello, I'm b.pl";

$ cat a.pl 
use warnings;
use strict;
use Capture::Tiny qw/capture_stdout/;
my $stdout = capture_stdout {
	require 'b.pl';
};
print "<$stdout>\n";

$ perl -I. a.pl 
<Hello, I'm b.pl>