If you're ok with converting the program's arguments into shell literals and combining STDOUT and STDERR,

my $output = `CVS -d :ext:.... 2>&1`;
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If not,

use IPC::Run3 qw( run3 );

run3(
    [ 'CVS', '-d', ':ext:....' ],
    \undef,   # /dev/null
    \my $out_fr_chld,
    \my $err_fr_chld,
);
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