my $num_subs = `perl -w -0777 -pi -e 'my \$c=s/foo/bar/g;print STDOUT "\$c\\n"' ./foo_test`;
die("Can't execute child: $!\n" if $? == -1'
die("Child killed by signal ".( $? & 0x7F )."\n") if $? & 0x7F;
die("Child exited with error ".( $? >> 8 )."\n") if $? >> 8;

chomp($num_subs);

##</code><code>##

my $num_subs = `perl -i -0777wpe'print STDOUT s/foo/bar/g' foo_test`;
die("Can't execute child: $!\n" if $? == -1'
die("Child killed by signal ".( $? & 0x7F )."\n") if $? & 0x7F;
die("Child exited with error ".( $? >> 8 )."\n") if $? >> 8;

##</code><code>##

use String::ShellQuote qw( shell_quote );

my $cmd = shell_quote("perl", "-i", "-0777wpe", 'print STDOUT s/foo/bar/g', 'foo_test');
my $num_subs = `$cmd`;
die("Can't execute child: $!\n" if $? == -1'
die("Child killed by signal ".( $? & 0x7F )."\n") if $? & 0x7F;
die("Child exited with error ".( $? >> 8 )."\n") if $? >> 8;

##</code><code>##

use IPC::System::Simple qw( capturex );   # Core module

my @cmd = ( 'perl', '-i', '-0777wpe', 'print STDOUT s/foo/bar/g', 'foo_test' );
my $num_subs = capturex(@cmd);
die("Child exited with error ".( $? >> 8 )."\n") if $? >> 8;

##</code><code>##

my $num_subs = 0;
{
   local @ARGV = "foo_test";
   local $^I = "";
   local $/;
   $num_subs += s/foo/bar/g while <>;
}