open my $PIPE, '-|', 'my-first-program', 'arg1', 'arg2' or die $!;
while (my $line = <$PIPE>) {
    chomp;
    # ... do something with the line
}
close $PIPE or die $!;
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You can output the processed data to the second program in a similar way:

open my $PIPE_OUT, '|-', 'my-second-program', @args or die $!;
# ...
print {$PIPE_OUT} $data;
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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G'day sewood,

Welcome to the Monastery.

If you're doing this on the command line, you can just pipe STDOUT from your first program to your Perl program and, in turn, pipe its STDOUT to your second program. Something like this:

$ os_prog_1 | perl_prog | os_prog_2
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See "perlop: I/O Operators" for full details of reading STDIN in your Perl code. Here's a highly contrived, minimal example which just reads input and passes it on unchanged (presumably, you'll want some processing to actually occur):

$ ls -l | perl -e 'while (<>) { print }' | wc -l
    821
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And, just to confirm the Perl code was passing on each line:

$ ls -l | wc -l
    821
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