my $regex = \b(.{1,50})\b(?i)('.join("|",@words).')\b(.{1,50})\b';
my $expr = qr /$regex/; 

# Later              
my $matchtext;
while ($text=~m/$expr/g){
  $matchtext.= "...$1<font color=\"FF0000\">$2</font>$3...<br><br>" ;
}