• get the include paths from $ENV{INCLUDE}
• split on /:/
• grep for the parts you want
• join the parts back together again:

print join ":", grep { m{^/users/ctbld/} } split /:/, $ENV{INCLUDE};
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However, you'll have a hard time manipulating environment variables from within a Perl program, so you'll have to do that as a one-liner

Update:

Damn it, its INCLUDE, not PATH