Module::Install has the can_cc function you can use in your Makefile.PL

use inc::Module::Install;

if ( can_cc() ) {
  # do XS stuff
}
else {
  # do pure-perl stuff
}

# rest of Makefile.PL
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$Config{cc} may be valid but not contain the name of any file on your system. eg, it could be 'ccache cc'.

The same question (presumably from the same monk) was asked on the module-authors mailing list: http://www.nntp.perl.org/group/perl.module-authors/2009/10/msg7927.html. There are several possible solutions posted there.

eg, it could be 'ccache cc'

FWIW, the C/Makefile.PL in Inline-0.45 seems to do a pretty good job of finding the C compiler (albeit rather kludgily).
I don't think it detects 'ccache cc', however.
Here's a C/Makefile.PL that should also detect 'ccache cc', though it has yet to be run by the intrepid cpan testers:

use strict;
use ExtUtils::MakeMaker;
use Config;
use File::Spec;

my ($cc, $exe) = @Config{'cc', '_exe'};
$cc =~ s/\s+-.+$//; #remove possible trailing options
my $comp = (split /\./, $cc)[0];

my $found = 0;
my $delim = $Config::Config{path_sep};

if ($cc =~ m|/:\[|) {
    $found = -f "$comp$exe";
}

# $Config{cc} might be something like '/some/place/cc'
elsif ($cc =~ m|/|) { 
    $found = -f "$comp$exe" || -l $cc;
}

# $Config{cc} might be something like 'ccache cc'
elsif ($cc =~ m|ccache|) {
    my @cc = split /\s+/, $cc;
    $found = -f "$cc[0]$exe" || -l $cc[0];
    if($found && $cc[1]) {$found = -f "$cc[1]$exe" || -l $cc[1]}
}

else {
    my $comp = (split /\./, $cc)[0];
    for my $lib (split $delim, $ENV{PATH}) {
    $found = -f File::Spec->catfile($lib,"$comp$exe") and last;
    }
}

print <<END;

Inline::C is packaged with Inline.pm because it is the most commonly u
+sed
Inline Language Support Module (ILSM). 

See also: Inline::ASM, ::Awk, ::BC, ::Basic, ::Befunge, ::CPP (C++), :
+:CPR,
          ::Foo, ::Guile, ::Java, ::Octave, ::PERL, ::Python, ::Ruby, 
+::TT, 
          ::Tcl and ::WebChat.

Config.pm indicates that your version of Perl was built with this C co
+mpiler:

    $cc

END

if ($found) {
    print <<END;
I have located this compiler on your system:

END
}
else {
    print <<END;
I cannot locate this compiler on your system.

You can install Inline.pm without installing Inline::C. But you'll
need to install another Inline language module (like Inline::Java for 
instance) to actually make use of it. 

If the aforementioned C compiler really is on your system, please make
+ sure
it can be found in the PATH and then try running this program again. O
+r if
you think I made an error searching for this compiler, simply answer '
+Y' to
the next question.

END
# '
}

my $answer = '';
my $default = $found ? "y" : "n";
while (1) {
    $answer = prompt ('Do you want to install Inline::C?', $default); 
    last if $answer =~ /^(y|yes|n|no)$/i;
}

if ($answer =~ /^(y|yes)$/i) {
    WriteMakefile(
                  NAME => 'Inline::C',
                  clean => {FILES => '_Inline_test'},
                 )
}
else {
    open MF, "> Makefile" or die "Can't open Makefile for output";
    print MF <<'END';
all::
test::
clean::
END
    close MF;
}
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Cheers,
Rob