You're not reading to EOF.

This:

my $s1 = <STDIN>;
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reads exactly one line. Since $/ is "\n" (see perlvar), your line ends at "\n".

If you want to read by line and get more than one line, I suggest some sort of iteration. Perhaps a while loop.

The most common idiom for reading a whole file line by line in Perl is:

while ( <$filehandle> ) {
    # do something
}
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In your case, that'd be:

while ( <STDIN> ) {
    # do something
}
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There's really nothing mysterious going on. All of this is well-documented and widely known.