/[^IMO]/
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Also, don't forget to "chomp" your lines, otherwise all lines will match...

This regexp will match a string that doesn't contain those letters:

/^[^IMO]*$/
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If that's not what you're after, please update your post after reading the guidelines in: How do I post a question effectively?

 print "$. : $line\n" if $line =~ m/[^IMO]/;

Look at the documentation for perlre, chomp, and perlretut. Others have given you actual solutions to your (very homeworkish) question.

A simple regex works fine, but transliteration is often more efficient. Here's an approach with tr///. This snippet iterates over each line of each file supplied at the command prompt, and prints the filename and line number, along with a highlighted version of the line's data showing where the problems are:

while( <> ) {
  chomp;
  if( tr/IMO//c ) {
    s/([^IMO]+)/[-->$1<--]/g;
    print "File: $ARGV, Line $.: $_\n";
  }
}
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This would be invoked as:

It could be expressed as a one-liner, in fact, like this:

perl -lnE 'if(tr/IMO//c){s/([^IMO]+)/[-->$1<--]/g; say "$ARGV-$.: $_"}
+' file1.txt file2.txt fileXX.txt
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The output is a little more terse, but it does the same thing. In either case, if your objective is just to print the bad lines, you can easily simplify the print statement.

How about...

cat file1 | perl -e 'while(<>){ next if !/IMO/; print $_;}'
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If the lines can only begin with IMO then:

cat file1 | perl -e 'while(<>){ next if !/\bIMO/; print $_;}'
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If case is not important then:

cat file1 | perl -e 'while(<>){ next if !/\bIMO/i; print $_;}'
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If you have multiple words to search for, then create another file named file2 and enter the search words:

vi file2
apple
orange
banana

grep -f file2 file1
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From the unix command-line... file2 is the list of words to search for from lines in file1... Note: you may have to run "dos2unix file2" to remove LF/CR or use chomp somehow...

Try this..

while(<FILE>)
{
 if($_ =~ /I/ || $_ =~ /M/ || $_ =~ /O/){}
 else
    {
     print "\n $_";
    }
}
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