For example,

print 'that\'s it',    "\n";
print 'that\'s it\!',  "\n";
print 'that\'s it\\!', "\n";
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that's it         \ escapes the delimiter (')
that's it\!       \ wasn't followed by the delimiter (') or \
that's it\!       \ escapes \
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Contrast with double-quotes:

print "\"that's it!\"\n";
print "\"that's it\!\"\n";
print "\"that's it\\!\"\n";
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"that's it!"      \ escapes "
"that's it!"      \ escapes !
"that's it\!"     \ escapes \
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