foreach my $line (@input){
    if ( /fred/ ){
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Is the same as:

foreach my $line (@input){
    if ( $_ =~ /fred/ ){
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You need to change it to:

foreach my $line (@input){
    if ( $line =~ /fred/ ){
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Thanks @jwkrahn for clearing this.

G'day Perl_Programmer1992,

Many of Perl's operations use $_ as a default value. You may have seen print and the for loop used that way:

$ perl -MO=Deparse -e 'for (0..2) { print }'
foreach $_ (0 .. 2) {
    print $_;
}
-e syntax OK
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[See B::Deparse if the code in that example is unfamiliar to you.]

$_ is also the default variable for a "pattern match (m//)". These are equivalent:

/fred/
$_ =~ /fred/
$_ =~ m/fred/
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Relating all of that to your code: by assigning the value of each foreach iteration to $line, you do not assign the value to $_ (either explicitly or implicitly by default); consequently, $_ is undefined when used as the default value in /fred/.

By the way, in case you didn't know, for and foreach are synonymous.

Thanks Ken for the explanation , this is actually a very important concept as I am still in the initial phase of learning perl.

Just another tip: there is no need to read all of STDIN into an array; you can iterate over each line as it comes in.

while ( my $line = <STDIN> ) {
    ...
}
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The diamond operator <> will do some pretty fancy stuff for you by default. You can for example leave out the explicit STDIN:

while ( my $line = <> ) {
    ...
}
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What is really cool about this is that now in addition to working with STDIN you can also pass a list of filenames to your Perl program and it will do the right thing.

See perlop - IO Operators for more details on the diamond operator.

Of course you also don't need to set an explicit loop variable for a program of this size; you can just use $_:

while (<>) {
    ...
}
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And at this point, you may want to look into Perl's -n command line option, which will automatically wrap your entire program inside that exact same loop:

bash:~$ perl -nE '...'
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Just another tip: there is no need to read all of STDIN into an array; you can iterate over each line as it comes in.

foreach my $line (<STDIN>) {
    ...
}
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The problem with that is that the foreach loop will read all lines into a list in memory first.

You need a while loop to iterate over one line at a time:

while ( my $line = <STDIN> ) {
    ...
}
[download]

Quite right, that's what I get for posting without testing my code :).

I edited my comment to correct this.

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