You have to create $lookfor at a broader scope. The {...} brackets effectively limit the variable's scope. Here is how you would make it available outside of the {...} block:

use strict;
my $lookfor;
if ( $line =~ /TC/g ) {
    $lookfor = "TC";
} else {
    $lookfor = "26";
}
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Update: Or you could use a different Perl idiom:

use strict;
my $lookfor = ( $line =~ /TC/g ) ? "TC" : "26";
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Note, in the snippet you provided, the /g modifier on your RE isn't doing anything for you.