#!/usr/bin/perl -w
use strict;
my @data = ('exception:mex',
            'qwerty',
            'tex:exception:mex',
            'exception:mex',
            'exception:tex',
            'tex:exception',
            'tex:exception:tex',
            'tex : exceptions:mex',
            'tex:exception:mex:tex',
            'asdf');
           
#desired is: tex:exception:mex           
#            tex:exception
#            tex : exceptions:mex

my @matches = grep{                        #approach #1
                    my @texes = m/tex/g; 
                    @texes <=1
                  }
              grep{/tex.*?exception(s)?/}
              @data;
      
print join ("\n",@matches),"\n\n";  

@matches = grep{ !/exception(s)?.*?tex/}   #approach #2
           grep{/tex.*?exception(s)?/}
           @data;

print join ("\n",@matches),"\n"; 

__END__
Prints:

tex:exception:mex
tex:exception
tex : exceptions:mex

tex:exception:mex
tex:exception
tex : exceptions:mex