One of the following "forking opens" might suit your needs

my $pid = open my $fhOut, "| the command ", or die ...;

## or
my $pid = open my $fhIn, "the command |", or die ...;

## or using the 3-arg form on *nix systems
my $pid = open my $fhOut, '|-', "the command ", or die ...;

## or 
my $pid = open my $fhIn, '-|', "the command", or die ...;
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Or list forms of the latter two, if you need to pass arguments, may make sense on your system.

See open and perlopentut for more information.