Generally in a case like this I use either BEGIN blocks or fully-qualified names, whichever seems appropriate. In this case the latter seems a better fit:

#!/usr/bin/env perl

use strict;
use warnings;

my ( $stdout, $stderr, $errcode );

if (eval { require Capture::Tiny } ) {
        print "Using Capture::Tiny\n";
    ( $stdout, $stderr, $errcode ) = Capture::Tiny::capture( sub {
                system "echo Hello, World | cat";
        } )
}
elsif (eval { require IPC::Run } ) {
        print "Using IPC::Run\n";
    no warnings qw{ qw };    # Otherwise Perl complains about the comm
+a
    IPC::Run::run(
        [ qw{ echo Hello, World } ],
        \'',
        '|',
        [ 'cat' ],
        \$stdout, \$stderr );
}

print "STDOUT: $stdout";
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